quicktime Version 10.4 (928.13) cannot open file error during save

This issue just started recently for me. When I searched, I found another user has the same problem. In Quicktime, I can record and save videos of less than a minute. Until yesterday, I could record and save much longer videos. Now, however, it starts to save and gives the error "Cannot Open File." This is reproducible. I found a proposed solution which gave a path to where Quicktime saves recordings, but there was no file there. I'd buy a screen recording app in the app store, but the reviews aren't promising for them. I'm running High Sierra 10.13.4 on a MacBook Pro. It's really annoying to make a long recording and then be unable to save it.