"The server responded with an error"

See my post under the same error message here and see if that helps: http://discussions.apple.com/thread.jspa?messageID=12199731&#12199731

I tried that solution, and I still get this error

The server responded with
“HTTP/1.1 500 Internal Server Error”
to operation CalDAVWriteEntityQueueableOperation.

I have the option to "Stay Offline", "Revert to Server" or "Go Online". Clicking "Go Online" just reproduces the error.

Is there another solution?

That sounds like something went wrong in the database the governs syncing.

Restore from your backup to restore your calendar.

Reset your sync history: http://docs.info.apple.com/article.html?path=iSync/3.1/en/9247.html and then follow the steps:

Go into iCal > Preferences > Accounts and delete the MobileMe account from here.

If you are using it you can then re-add it and be careful to enter in the correct name and password.

Remove the following to the trash and restart your computer:
Home > Library > Caches
Home > Library > Calendars > Calendar Cache, Cache, Cache 1, 2, 3, etc. (Do not remove Sync Cache or Theme Cache)
Home > Library > Preferences > com.apple.ical (There may be more than one of these. Remove them all.)
__NOTE: Removing these files will remove any shared (CalDAV) calendars you may have access to. You will have to re-add those calendars to iCal > Preferences > Accounts.

Once the computer is back up and running open iCal and test.

I have had the same problem since last week. I tried all the steps and it didn't work. I also got a similar server error message while trying to modify existing events at me.com. I suspect the problem is on MobileMe side.

Easy way to find out if the issue is computer side or server side is to create a new user account, add your MobileMe account, and test iCal: http://docs.info.apple.com/article.html?path=Mac/10.6/en/8235.html