Thanks @john_dennis

Does anyone else have any ideas? - I only have 1 more day to test.

Using purchased Vegas Pro 19 b341 with just some random events on the timeline. After hitting Ctrl+M, I had to choose a render template - I chose MAGIX AVC/AAC MP4. The Ctrl+M render worked:

And it auto put the render on a higher track:

Is it possible that the test version does not allow selective pre-rendering?

Trial versions of Vegas Pro usually won't render if the total length of the project is 2 minutes or more, but from the screenshot, the project on the OP's timeline is less than 2 minutes in length. A possibility is that there is something at the 2 minutes point or later on the timeline - like a marker - that makes the project 2 minutes or longer in length.

@Dexcon
Thanks for testing!
Ctrl+M (render to new track) also works for me, exactly as you described.

However, I wanted to try Shift+M (selective pre-render) as this is so praised for version 19 because it is supposed to be very fast and a very good replacement for Shift+B.

@walter-i.  ... Oooops. I had Ctrl+M on my mind and misread your post. Sorry about that.

I've gone back and tested Shift+M, and it works perfectly - and very quickly (no FX etc used). I put BCC Flicker Fixer on an event (BCC-FF usually causes timeline fps playback to stutter a bit on my computer) and then used Shift-M. The selective pre-render took much longer than without any FX, but playback with Shift-M was steady at 25 fps.

@walter-i.
Did you change any of the Selective Prerender settings in Project Properties? How about the given path?

And did you try using internal Generated Media instead of video clips for a test?

@walter-i.
Did you change any of the Selective Prerender settings in Project Properties? How about the given path?

And did you try using internal Generated Media instead of video clips for a test?

@Marco.
Thank you for the response
Unfortunately, my test phase expired today.

I did not change any prerender settings in the project properties - and unfortunately did not use any generated media instead of video clips for a test.

Nevertheless, I will buy version 19 soon, as this was the only problem I encountered - all other tests with the functions I use regularly - were successful.

Today I bought Vegas Edit 19 in the 365 version.
I have done many tests - sometimes it works, sometimes it doesn't - haven't figured out why and why not.

@walter-i.
Did you change any of the Selective Prerender settings in Project Properties? How about the given path?

I tried it both with the default - path settings and with a modified path.
It is interesting that the path and resolution does not change again after a full reset. (Including deleting the cache)

And did you try using internal Generated Media instead of video clips for a test?

Yes, I tried it too - as I said, sometimes it works, sometimes it doesn't - but the chances of it working are only 10%.

Here is a simple *.veg with content from the media generator and a crossfade if anyone wants to test it:
https://www.dropbox.com/t/q5PHtZ9WTl8dX4hB

PS:
The installation went through without problems (build 381).
The graphics drivers Nvidia Studio 472.12 and Intel 30.0.100.9805 have been updated.

@walter-i. Prerendering (Shift+M) your veg works as expected, no issues on my system.

Also here on laptop no problem in VPro19 b. 381
Look to the video taken also with VPro 19 381/File/Vegas capture: