SEP 11 + Citrix (on 2008r2) = hang on shutdown.

I have a customer with two Citrix "terminal servers" running on Server 2008r2. Starting this past week, our daily reboot at 2AM has been failing on shutdown. Last thing logged in the event log is various Symantec AV-related services stopping.

I uninstalled SEP, rebooted (still hung - had to force reboot the server). Rebooted after that, it did so just fine (repeated 3x).  Re-installed SEP, rebooted another 3 times, all seems fine.

Any known issues with SEP11/Citrix/2008r2 that would provoke such a behavior?   (The boss asked fro me to fix the problem, which I did, but now wants to know for the customer if Symantec knows what it might be... I know, I know, slightly difficult to diagnose after it's no longer happening...)