Can't synchronize calendar with Thunderbird 78.5.1

[here-abarany]

I've been getting numerous errors with using the calendar with Thunderbird 78.5.1 and DavMail using Office 365. Modifications aren't saved, including dismissing reminders, and every time it synchronizes it gets a MODIFICATION_FAILED error. The console output I see in Thunderbird is:

console.warn: Lightning: [calCachedCalendar] Unable to perform playback action delete to the server, will try again next time (040000008200E00074C5B7101A82E0080000000050FBC932D4D2CD0100000000000000001000000042FC18707019D1449F3876169A1C9711,Server Replied with 503) console.warn: Lightning: There has been an error reading data for calendar: Work. However, this error is believed to be minor, so the program will attempt to continue. Error code: 0x80040111. Description: Server Replied with 503

I tried looking through the DavMail logs but it scrolls by too fast to see any individual message. I did see a 503 error reported in there, but it scrolled away too quickly to see any context that might be useful.

[here-abarany]

I'm able to work around this issue when using the TbSync DavMail extension in place of the built-in DavMail support in Thunderbird.

[here-abarany]

It looks like that was only a temporary solution, it now gets the same errors with TbSync as well. I pretty much have to disable the calendar at this point and use the webmail version instead.