FCPX Viewer window blank

I am working in FCP 10.4.8 on Mac OS 10.14.6. I have some crucial software that only exists in 32-bit, so I'm staying away from Catalina for now. Everything has been working fine until this morning.

Today, my viewer window isn't showing any video. It was working fine yesterday. I have changed nothing in my settings, and I am working with the same footage from the same camera with the same in-camera settings I always use (Panasonic GH2). I shot some footage earlier this week and edited it successfully. Today, I opened FCPX, closed the library from yesterday. Created a new library and imported footage from that same camera and card to start a new project, but the viewer window is black, regardless whether I'm skimming, playing back, or sitting still on a clip. I re-opened the other library that was working, and now it's not working either.

I have made sure I am viewing all channels in the drop-down (not just alpha) and have switched that setting back and forth, just to be sure.

I have restarted FCP and my Mac several times each, including one time with option/command held and reset prefs.

I opened old libraries and tried to view old clips and timelines that were previously working — nothing, just black.

I can hear the audio, and when I pull up scopes, they show up and function, but there is no video image behind them.

I have switched back and forth between Better Quality and Better Performance.

I output (shared) a master file from the timeline and the resulting videos works fine in Quicktime, but also does not show up in the FCP viewer. It's as if the viewer is just dead.

Any suggestions?