iMovie stopped exporting

I've been using/learning iMovie in order to create online classes for my students because of COVID19. Everything has been working fine until this morning. iMovie just won't export a file. I've created and exported about 30 videos thus far and I understand the basics. But now, when I select all the clips in a project and select the "Share - File..." option and hit "OK", nothing happens. No error messages, nothing. The little pie-chart progress indicator flashes for a fraction of a second before disappearing for good. The application doesn't bug or freeze (as I can keep working on the video or move to other projects), it just doesn't export the file. I also can't export any of the videos it previously exported in the last few days and weeks. Nor will it export and brand new video. I tried deleting the preferences: didn't solve anything. I'm on 10.13 and iMovie 10.1.6 (and I can't upgrade). Thanks. P.S. The only change I made to the computer since yesterday is that I installed iOS 14 on my ipod touch. I had to use the MobileDeviceUpdater in order for iTunes to recognize my ipod touch. I can't imagine why it would change anything for iMovie, but nothing else changed.