Lightroom Sync Loop + thousands of errors with 7.5

Sync has generally worked well for me in the past but now it has gone completely mad.

Every time I open Lightroom it says "syncing 15,000 photo" and basically it tries to sync everything again. Then when it gets nearly complete, the number starts climbing again and so does the "sync errors" number until you end up with thousands of sync errors.

While it does this, Lightroom basically becomes unresponsive and often crashes and requires a Force Quit, at which point it then starts all over again.

I'm running LR 7.5

While the Sync is running, the sync panel in the preferences menu shows nothing at all. There are thousands of images now in the "sync error" collection, but nothing shows up in the sync activities section so I have no idea what error message it thinks it is getting.