I am seeing this exact thing currently with latest studio drivers from NVIDIA and latest VP19 550. After a number of BRAW files added ~20 in my case, things go black and Vegas stops working correctly for video (preview, timeline, trimmer, scrubbing thumbnails all black).
What appears to be happening is that when opening BRAW (6k, 24p) files, the GPU (NVIDIA RTX laptop 3070) memory is loaded up incrementally with each one - GPU has 8GB RAM and once that fills up (monitoring memory used with GPU-Z app) any additional added BRAW are black. The larger problem is that the memory never seems to release/clear, so once I open about 20 BRAW files, my 8GB is maxed and everything else after is black. Then also the trimmer, timeline, preview etc. also all go black because, I assume, the GPU can't function properly since its memory is full.
Even if I remove all the clips from the timeline and from the project, the GPU memory load never decreases until I quit VP19. I don't expect this is the preferred behavior.
In further testing, I changed 'GPU acceleration of video processing' to the Intel UHD and there is no black frames - but GPU-Z does show large amounts of system memory being used by the Intel GPU.
My issue is 100% reproducible. It could well be related to the NVIDIA as I mentioned. I see you have an AMD GPU. Also, as I mentioned above, when I changed 'GPU acceleration of video processing' to the Intel UHD vice the NVIDIA, the issue went away. I've been editing that way to get the project done. However, there is clearly still an issue with the NVIDIA GPU memory full causing this error.
I appreciate you checking it as well with the AMD GPU which seems to be working fine per your posts. (Unless you have your GPU acceleration of video processing set not to use your AMD GPU.)
My hope is that since this is an error and is reproducible that the memory management issue can be isolated and addressed for a future release. I would like the option to use my NVIDIA GPU.