HTTP 501 (Not Implemented) error could happen for different reasons, and we would request you to please send us client request URL (that returned that error message) with exact date + time (with timezone) of when that error message occurred by email to firstname.lastname@example.org (PGP key).
Meanwhile, just to confirm that it is not happening due to a corrupted client cache, you can empty your CalDAV client cache. Please find instructions specific to your CalDAV client in its online documentation or through its support. In general:
- Backup your CalDAV client data, particularly if you have locally stored data.
- Delete cache files in /Users/$USER/Library/Calendars/ — basically files with the word “Cache” in the title.
- Then open CalDAV client and try to connect again.