Sorry, the forum blocked me from posting too frequently again and I had to wait until today. I have the app running again now - it is still trying to sync without success, no errors in the console.
I could zip together a collection of suspect images, I wasn’t able to isolate it to just one - after removing all of the notes in the notebook where the issue started, the sync still seems to be stuck even though they are gone (a cache corruption or similar?).
Unfortunately I cannot test this any further for now - my plan was to migrate to inkdrop over the weekend and then subscribe, but the week has started again and I’ve had to cancel for the time being! Apologies for not being able to solve this!