Lightroom export issue

I'm on a PC using Win10, Lightroom Classic, most recent update installed (12.3). I've been using the export feature for the past 2 days to watermark a large number of images. The images are in discrete folders and I'm watermarking in batches from the individual folders. Everything was great until suddenly it wasn't.

1. There are numerous discrepancies between the number of source files and the watermarked output files in each folder.

2. Rather than try to pick out which of the undreds of images were missed, I deleted the entire batch from the mismatched folder and tried to re-export; nothing happened. None of the images selected exported.

3. Clicking "export" simply closes the dialogue window; there is no progress bar.

All the source images were created in LR and have not been moved; LR knows where they are, and there are no ! on any of them.

To be clear: the program was working perfectly until about 2 hours ago.

I've restarted the program several times.

I've rebooted several times.

I have the latest LR version (12.3).

I've run my computer maintenance software

Any ideas?