Has anyone been seeing errors of this sort in iCal?

Basically, iCal goes into offline mode with/without notice. When you try to go back online, you get an alert; the only solution is to quit/restart iCal.

One user copied down her error message:
Request for calendar failed.

The server responded with
"HTTP/1.1 500 Internal Server Error"
to operation CalDAVRevertEntityQueueableOperation.
I've also seen alerts offering to revert to server data, but I haven't noted the exact wording. In any case, clicking "okay" to revert to server doesn't do anything--you still have to quit & relaunch iCal.

The user in question is still using Mac OS 10.5.6 due to Bug 38112 – Creating appointments in iCal does not send the invite but once 5.0.17 is released, I'll upgrade her to 10.5.7. However I'm pretty sure I've seen the error under 10.5.7 as well.