I'm also getting this error message multiple times a day. I click on Go Online and MobileMe syncs properly until the next time it pops up. I tried deleting my account in iCal and recreating it, but that didn't help. I participated in the CalDAV beta a few months ago, so I don't know if MobileMe is confused when it authenticates to sync to the new production server, but this error message is definitely becoming annoying.
I have three Macs, and all are experiencing this error message repeatedly daily. At this point, it seems to be happening hourly when it checks with MobileMe to see if anything has changed, even through Push is activated. At this point, I have no idea how to fix this. My MobileMe credentials are correct, but the server is apparently rejecting them until I tell it to try again when the error message pops up.
This just started happening over the past couple of weeks, more frequently as each day has passed, now happening hourly.
Does anyone have any ideas why this is happening, or how to fix it? An Apple Advisor suggested that I have a corrupted repeating event, but iCal doesn't give me any way to find out if that's true by identifying a bad event.