Changing the calendar an event belongs to on Fantastical 3.3.8 on macOS 11.2.3 results in HTTP `500` and blocks future changes from syncing

[0xedward]

Steps to Reproduce

1. Followed the steps here to install etesync-dav with SSL on macOS 11.2.3
2. Add at least two calendars to EteSync account (e.g. CalendarA, CalendarB)
3. Add Other CalDAV Account in Account Settings on Fantastical 3.3.8
4. Suppose we have EventA on CalendarA, select EventA on Fantastical and edit EventA so that the calendar field is CalendarB.
5. We expect EventA to move to CalendarB with everything being identical except the calendar it belongs to.
6. But instead, the next time Fantastical syncs with EteSync, we get the following error:

Sync error (Personal)

Fantastical encountered an error while communicating with the server.
Error: Request failed with status code 500.
https://localhost:37358/etesync-user/randomstringofchars/anotherrandomstringofchars.ics

7. Clicking Retry button on the error prompt from Fantastical will fail with the same error.

This will block any future EteSync changes from syncing from and to Fantastical. For example, if we create a new event (Event C) on CalendarA. EventC will not show up in CalendarA's journal.

On EteSync side, if we look at the journal for CalendarA after moving EventA from CalendarA to CalendarB. EventA will have its name changed to Not found and you will be able to load the Raw and Revisions data, but the Main data will be completely empty. The journal for CalendarB will not contain EventA either.

This issue has the same error prompt as this one reported about Google Calendar and Fantastical. Has anyone else ran into this and happens to know a fix for this?

[solarkraft]

This looks a lot like what I run into with Apple Calendar, except that it doesn't mess up the original event. The error message is The server responded with “500” to operation CalDAVMoveEntityQueueableOperation..

[solarkraft]

Oh hi, past me.

except that it doesn't mess up the original event

Never mind. Choosing "revert to server" often deletes the whole thing. It happens on every second or so operation and is driving me crazy.

[solarkraft]

Could be related to #109.

[solarkraft]

Is very likely related to #183