I had this same issue. I’m confident it is caused only by a lack of disk space, after removing a big video file, it immediately started working again.
Once I figured this out I realized that I’ve had this issue before, about a year ago. That time I also spent a few hours, very annoyed, not being able to find the cause of this issue.
I would very much appreciate it if the app would give any feedback on this. Why is there no error produced? I use the app so sparingly (usually it “just works” so I never have to open it) that I didn’t even notice there were no “progress bars” when refreshing the calendars. I skimmed through the debug info/logs, but did not see anything about disk space mentioned there.
I would expect some kind of feedback when it can’t sync the calendars, right? I understand that in this case it probably succeeds in connecting with the server, but then fails when it’s actually downloading the new data, or handling this. But there should also be a check in place for that right? And a error message that gives some clue as to what has gone wrong?
The calendar apps I use on my android phone both did show some appointments, but the appointments were a few weeks out of date, and did not match what I saw in thunderbird on my laptop, or in the webinterface to my calendar software. I guess that is still stored in the android calendar database?