Nothing wrong with all.
Did you tried to place the rendered file back on the timeline of an empty VP18 project and looks how it plays?
This looks to me a VLC problem.

Further, make no changes in the settings of Options/PreferencesVideo and Options/Preferences/File I/O because build 373 make on its own the best settings for these at installation.
To make this healthy again with you do a complet and exact Rest of the program following this tutorial exactly: https://www.vegascreativesoftware.info/us/forum/faq-how-can-i-reset-vegas-pro-to-default-settings--104646/

Change nothing after that and controle if you have the best GPU driver by checking and following this

@deckingman It’s possible that VP18's QSV encoding isn’t exactly the same as your SP16, or maybe you didn’t encode using QSV in SP16? Make sure that you have the latest graphics drivers, especially the Intel ones for your QSV encoding. Try rendering out using only CPU, not QSV, if updated drivers don’t help.

Ok, I see you already have the latest Intel driver.

If possible, maybe make available for download a sample “source” and “rendered” out clip, that has the issue, that we can check out.

I agree with trying a second player (or even just importing to Vegas again) to confirm that it is the file that is defective.

If it's the file, try turning dynamic ram preview to 0 for testing purposes. Then try turning preview GPU acceleration off (re-enable before editing again).

Thanks guys. This is very weird. Fairly sure it's not VLC as previous MP4s that I rendered using SP16 all play fine. Windows media player chokes, gets pixelated and crashes on that Pro18 MP4. Dropping the file into a new VP project does very weird things. It plays fine up to about 1 minute 03 (just like VLC) but then the screen goes blac but the audio continues. Eventually the screen re-appears but the audio is then out of sync. What is really weird is that if I then stop playback of this MP4 in VP Pro, then start playback again from the beginning, the video starts from the beginning but the audio resumes from when I stopped playback. Then the audio gets stuck in some sort of weird loop and repeats. The only way out of that is to delete the MP4 and load it again.

Unfortunately, if I render just a short section of the video, it plays fine. So I've uploaded the entire thing to my Google drive. Here is a link https://drive.google.com/file/d/13Jz09LbKTo3u2YBdPuHw8Kp-N3SHhkJq/view?usp=sharing. For me, on my PC, the problem starts at 1:03 through to 1:32 then the rest plays fine.

There isn't a single source - the video is made from numerous clips - a pre-rendered intro followed by numerous video clips and stills with separate audio tracks, so I'm not sure what information to provide regarding the source.

I guess what I should do is a complete reset following @j-vs advice.

Could the fact that I don't have a separate GPU be a factor? There wasn't anything in the VP18 spec which indicated that it is a pre-requisite.

@deckingman Did you try the two pieces of advice I suggested? [Edit, I see now you have GPU preview acceleration disabled. Now try dynamic ram preview = 0MB] That's what Magix recommended in the past when I had render errors. Resetting Vegas settings can be helpful if it's far from defaults.

Is there a reason you are using legacy HEVC and AVC decoding? Is File I/O decoding set to auto?

What happens when you don't use the Intel QSV mode and just do CPU only?

For the source, maybe supply what type of media is at the point when it starts to display wrongly. If it's variable framerate cell phone footage you might want to transcode using software like Handbrake (there are other threads on this in the forum) and then bring back into Vegas ("replace media.")

No, lack of dedicated GPU should not be an issue.

I'm trying to download the file but it's going to take a while at 2.3GB! I am using Media Player Classic.

@deckingman
I downloaded your HEVC renderfile and I see the same as you on different players and also on the timeline of Vegas. There I see that the problem is caused by a frozen frame for that amount of time. If you did it not yourself one way or another, is has to be your GPU driver for the Intel 630. Therefore I showed you already the way to install the latest driver nr. 27.20.100.8935.
Let's see how it goes after resetting the program and installing that latest driver for using the QSV option in Magix HEVC.
With me on laptop and desktop from signature I render the same files as you use to Magix HEVC with QSV without seeing problems.

@RogerS Yes, I think I've established that it's the file and not VLC. TBH, I'm confused by your second piece of advice. Preview is just fine - there is no problem when I render the video, it's just the final result that has problems with playback. GPU acceleration is turned off - I don't have a GPU as such.

So, I've reset VP to all it's default values and cleared any cached data. I've double checked that the graphic driver is the latest version. The problem persists. I've managed to replicate the problem by rendering just the first 2 minutes of the video. I used MAGIX HEVC/AAC MP4 - Internet HD 1080p 50fps (Intel QSV) without any changes to the default template. I've uploaded this render to a folder on my Google Drive - it's a much smaller file that the previous one. I've also uploaded a sample of source footage. This is not the exact same clip as the rendered video, because that used part of a much bigger clip which is a very big file. Link to that folder containing the MP4 and the source sample is here. https://drive.google.com/drive/folders/13dzACKc7_LWsogHkR0-ie-JbH1RIU_j7?usp=sharing

I can confirm the file isn't working from 1:03 to 1:32- it's frozen with no audio using MPC. Odd as it's just the same clip that works before and after it.

Can you trying setting dynamic ram preview to zero MB (should be at 200MB by default)? We aren't talking about preview here, we're talking rendering, and this setting is known to causes issues with renders, which is why I recommended it twice already. Again, this is what Magix advised me when I had render errors.

Can you render without QSV? Change that to Mainconcept or whatever else the CPU option is. QSV rendering isn't particularly high-quality, anyway.

This is my driver

Can you trying setting dynamic ram preview to zero MB (should be at 200MB by default)? We aren't talking about preview here, we're talking rendering, and this setting is known to causes issues with renders, which is why I recommended it twice already. Again, this is what Magix advised me when I had render errors.

Can you render without QSV? Change that to Mainconcept or whatever else the CPU option is. QSV rendering isn't particularly high-quality, anyway.

Sorry - remember, you are dealing with an oldie here. :) I'll do exactly as you suggest.

The Intel 630 is a GPU, by the way, and you could use it to speed up timeline playback:

Not that that helps you with this issue and if you're having render problems keeping it off is advisable.

It's also likely active here in file I/O where it helps decode supported video types (thought not MTS files I believe):

I just tried to render the test mts footage you have as 1080p HEVC using the same template you used using QSV (which is my Intel 630). From what I can tell it worked fine (no pauses). That doesn't really say much, though unless you also can't render this particular clip either.

Can you trying setting dynamic ram preview to zero MB (should be at 200MB by default)? We aren't talking about preview here, we're talking rendering, and this setting is known to causes issues with renders, which is why I recommended it twice already. Again, this is what Magix advised me when I had render errors.

Can you render without QSV? Change that to Mainconcept or whatever else the CPU option is. QSV rendering isn't particularly high-quality, anyway.

Sorry - remember, you are dealing with an oldie here. :) I'll do exactly as you suggest.

No problem at all. Vegas works in non-intuitive ways at times, and the only reason I understand it is because I had to troubleshoot my own problems in the past. It's just an educated guess.

Addendum: driver looks good and is the same version as on my system with the 630.

it's just the final result that has problems with playback. GPU acceleration is turned off - I don't have a GPU as such.

You showed that you have one, the Intell 630 with the latest drivers. And when it is not automaticcally chosen at the install of VPro 18 you have a problem with your BIOS in which it is probably switched off.
You have to dive into the BIOS to make it visible or let your computerspecialist do it for you.
This is what I see in VPro 18 with the Nvidia switched off

and this is in my Control Panel / Device Manager / Display Adapters

Quick update. I've changed dynamic ram preview to zero and rendered the first 3 minutes using MAGIX AVC/AAC MP4 - Internet HD1080 50 fps (non QSV). The resultant MP4 plays fine. So I'll render the entire 18minutes or so using these settings and report back. It'll likely take an hour of so to render as it seems to take 3-4 times as long as HEVC.

it's just the final result that has problems with playback. GPU acceleration is turned off - I don't have a GPU as such.

You showed that you have one, the Intell 630 with the latest drivers. And when it is not automaticcally chosen at the install of VPro 18 you have a problem with your BIOS in which it is probably switched off.
You have to dive into the BIOS to make it visible or let your computerspecialist do it for you.
This is what I see in VPro 18 with the Nvidia switched off

and this is in my Control Panel / Device Manager / Display Adapters

Sorry, I meant that I don't have separate GPU - just the one that is part of the processor or motherboard or wherever the dam thing is. 😀 I don't have an additional video card is what I meant.

It is indeed enabled as you describe.

While I'm waiting for this to render out, would a separate graphics card help much? If so what would be a good choice? Being a pensioner, I don't have a great deal of cash to throw at it - say £200 or maybe £300 as the absolute maximum.

It is indeed enabled as you describe.

And was it set off at the installation of Vegas 18??

It is indeed enabled as you describe.

And was it set off at the installation of Vegas 18??

No. It's set to on by default.

For HEVC you can also render without QSV. Just change the encoder to NV Encoding. Maybe save this setting as its own render preset:

I wouldn't spend money if you don't have to. For rendering, while GPUs (NVENC, QSV, VCE) are faster there is a loss in quality if it matters for what you are doing. Personally, I use the tool Voukoder within Vegas to render faster than Vegas can do internally using CPU only. You can try it yourself (it's free, donation requested) at https://www.voukoder.org/

Download the connector and Voukoder and install both. Then select Voukoder in the render settings and you can go into its menu to pick how you want to render the file. Note that resolution is set by your project properties, not within Voukoder.

If you are in a hurry for renders, something like the NVIDIA 1660 or cheaper 1650 should be good enough and not that expensive. Just doublecheck it will fit in your system, your power supply is ready for it, etc.

No. It's set to on by default.

Why did you turned itoff writng this: "Other Vegas settings-GPU acceleration is turned OFF" in your Opening Question?