Apple Calendar Write Error

Try the following first. If this doesn't work, it's likely a server issue. Contact the admin. 

Error: CalDavWriteEntityQueueableOperation
Response: 500

How to fix:

Apparently caused by corrupted Calendar caches and it occurs in macOS.

Quit Calendar

1. Open Activity Monitor and Quit "CalendarAgent"
2. Make sure processes are gone: Open Terminal $ sudo killall CalendarAgent

Clear cache using Terminal

1. $ rm ~/Library/Calendars/Calendar\ Cache
2. $ rm ~/Library/Calendars/Calendar\ Cache-shm
3. $ rm ~/Library/Calendars/Calendar\ Cache-wal
4. Launch Calendar