I have been using my current setup for a few years now and haven't ran into this problem before. Whenever I try and rip a 4k disc (the bluray that comes with works fine), the output files do not work. Makemkv does not notice any errors and says every download was completed successfully.
What I notice, is that my output file is smaller by a few GB, even the extra files are off by a dozen or so MB. There is no thumbnail, and none of my video players can play it. I tried a previous 4k disc I already ripped with an output file that is playable, and the issue persists. I have downgraded makemkv to known usable versions, and nothing. I have tried to make an unencrypted backup and an encrypted backup, both result with the same issue.
The only thing that I have done on the PC since last known successful rip, was to add another SSD and try out SteamOS on it, which remapped my external optical drive to a new letter - I switched back to windows for makemkv and handbrake.