you can "solve" this issue doing the following:
1) In the error dialog select the keep offline option.
2) Copy the Event
3) Reconnect to the server and the in the error dialog select the revert to server option.
4) Paste the Event
It is f* boring, but works. I hope that the new Lion iCal come with this solved.
"one more thing": there will be 2 events on the server but with diferent calendars
I am having a similar problem.
The mystifying part is that the error happens on one computer, but not on another. As far as I can tell, the settings are the same on each machine. The error appears on an iMac on the same local network as the server. It does not appear on a MacBook Pro, either on that same network or remote.
Any ideas about what might cause different behavior on different machines?