“403” to operation CalDAVWriteEntityQueueableOperation."

I have started using a server and moved my local iCal calendars to the one on the server so I can better synchronise them. But the day after I did that I get this error in iCal:

"The server responded with an error.

Access to “event from years ago” in “the second added calendar” in account “local” is not permitted.

The server responded:

“403” to operation CalDAVWriteEntityQueueableOperation."

[Ignore] [Try Again] [Revert to Server]"

Each event starting with the beginning of that calendar (years ago) seems to generate an error. It will take a day to click on Ignore for all those events...

Any suggestions on how to deal with this and how to prevent it?