The problem is thumbnailing the files. We've been using FFMpeg for a while to open files when they are first loaded. If for some reason that fails, we try to fall back on Quicktime. This can happen for instance when FFMpeg doesn't recognize the codec.
On 4.2, we made a silly mistake where DXV files would not open with FFMpeg. As long as you have Quicktime installed or aren't trying to load new files, you don't notice. Playback is still hardware accelerated. If you don't have Quicktime installed, the file also plays as normal, it's just that blank thumbnails are created.
We're bringing out 4.2.1 shortly to address this and a few other issues. In the meantime, the quick solution for the thumbnailing is to install Quicktime and clear the thumbnail cache:
https://resolume.com/manual/en/r4/locations
If you are having problems with a file with an unknown codec, we'd love to take a look at it.