I strongly second what John wrote. 21.300 has many playback bugs- for 21 I'd stay with build 208.

Your signature says Vegas Pro 21-300. Why not Vegas Pro 315 or Vegas Pro 21-208.

I refused to even download 300.

Because in newer version there is EVEN MUCH more bugs. And my footage is even worse to work properly. There was a thread in this forum... I had a newer version and your tried to help me with that. I had to refuse never version for my files to start working.

@Vegas21_user Some time around the VP21 era I had one old MP4 AVC file that opened similar to your pic, all my other files were ok. So assuming it was a file problem I Rewrapped it using ShutterEncoder, I didn't change any settings so in my mind it didn't change the actual video it just put it in a new MP4 container, the file worked then.

Hi Gid! The problem is not with a single file, it's with EVERY file. Vegas has gone crazy and won't read any 4K file that it imports.

Converting to another format is not the answer. If you want to approach your work professionally, you need to use software that works. Unfortunately, Vegas has never been the best choice for professional film work.

@Vegas21_user Hi.

Rewrapping isn't converting.

I've been reading almost every post on this forum for about 5yrs & people do have problems with their files or Vegas but I don't remember anybody else having problems like yours, If I'm wrong can you point me towards similar posts?

Rewrapped?

I've never do that. Always converting (which I hate, too much time was taken).
Bur even a rewrappind causes us to have to WAIT. It takes some time. Like convertion.

Converting takes too much time. So I want my file to work immediately after import from the camera. It has to be that way.

I have an update on the subject. A strange update. I noticed that it's not the 4K file that's causing the problems, but the 1080p file that's starting to cause them (my timeline consists of 4K and 1080p files - from the same camera).

Interestingly, when we select the preview mode to "Preview", the glitches are not visible (maybe because it uses proxy preview). But when we set the preview quality to "BEST", errors appear. In the video below, I show how Vegas 13 behaves (there are no errors) and how Vegas 21 behaves (here there are errors).

Here I switch between Vegas21 and Vegas13, showing how it looks for me:

This is exactly the clip that's causing the problems: https://we.tl/t-lyuRjXk3W6

Here's an example of rewrapping- I'd try that before converting as if it works you're done in seconds.

I don't think that would help with 5DMark IV mp4 footage though as it seems to have good metadata. You can see what's different between the 4K and HD files using MediaInfo. It may be more than just the resolution.

VP 13 used compoundplug, VP 15 to 21.208 used so4compoundplug (when legacy AVC unchecked) and VP 21.300-present use MxCompund to decode this type of file.

For 21 you could try enabling the legacy AVC decoder or turning off the hardware decoder in preferences/ file io.

The glitching is an issue others reported and got fixed already. I just tested it in 22.243 with my NVIDIA laptop and it's fine. I also tested it in 21.208 and it's fine. 21.300 is a known problematic build of VEGAS that was quickly replaced, so use it at your own risk. It's not in development any longer and won't be fixed.

@Vegas21_user Same here, in VP21 (315) & 22 (239) that file is fine, I duplicated your 20 times, long & short crossfades no problem. Different Preview options Good (Full,Half) etc. no probs, I added 20 other 12920x1080 files, no problem.. I added transitions, fxs .. no problems, sorry don't know what else to try.

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I don't understand your comment, Like I say 'Rewrapping isn't converting' & it literally says 'without converting' on the first image of the video.

Oh, no! It STILL takes some time. I tried that and STILL I have to wait for that file being "converted".

For 21 you could try enabling the legacy AVC decoder or turning off the hardware decoder in preferences/ file io.

I rember that all of my clips were not working while doing that earlier. I know that this is strange, but that is what it looks. Here is what will happen if I disable/enable options form "File I/O":

A footage is not possible to edit (black screen).

PS
Even in that earlier topic you said "switch to build 208".😊😉😉

For 21 you could try enabling the legacy AVC decoder or turning off the hardware decoder in preferences/ file io.

I rember that all of my clips were not working while doing that earlier. I know that this is strange, but that is what it looks.

You need to dig a little deeper, try it again and right-click on your media and see which decoder it is using (so4compound?). If neither legacy AVC nor turning the hardware decoder off help then it's a problem with the video engine, not just the decoder. That means you CANNOT use build 300 for this type of media and there is no reason to continue to test it and waste your time. It works in 208.

Your added video doesn't tell me which options you changed in file io.