Quicktime / Handbrake suddenly not working

Hello! I'm having two strange problems. Although I'm not sure they're related, they appeared at the same time and both concern video.

Today I couldn't open any video file that I've tried in quicktime. I've tried both .mov (Prores 422) and .mp4 (encoded in handbrake with h264). They all work fine when opening them in VLC. They also work fine opened from the same external disk on another imac, both in quicktime and VLC.

Also, I can't compress videos in handbrake, which worked fine yesterday. It only says in the log: "scan: unrecognized file type", when trying to compress Prores 422 to h264 or h265. The same process (same external disk, same files) works fine on the other imac I tried it on.

Does anyone have an idea of what might be the problem here? Thanks in advance