Outlook authentication problem after upgrading from Exchange 2010 SP2 to SP3 plus roll up

Upgrading an Exchange 2010 SP2 server to Exchange 2010 SP3 + RU6 in readiness for Exchange 2013 migration.

The service pack was applied, and then the roll up. No problems.

The next day some Outlook users were complaining of an authentication prompt 'randomly' asking for username and password when they were doing nothing in Outlook. After much investigation we tracked it down to the offline address book. This explained why the authentication prompt was only appearing infrequently, when Outlook went to download the address book in the background. After double checking the OAB physical directory / virtual directory permissions etc and oab.xml could be opened as you would expect there was no obvious cause to this error.  Terminal server users did not have this problem as they were using non cached mode without the offline address book. Changing a cached Outlook user to direct would also sort the problem, by bypassing the offline address book in the same manner. My colleague suggested we reboot the server, but this was more in desperation than anything else. The server had not been rebooted since the application of the SP3/RU6.

The server was rebooted and... magically the prompt went away. How bizarre is that? When you first install Exchange the setup programme tells you to reboot before putting the server into production, but not the service pack. The moral of the story is to always reboot after applying a service pack. I have no idea what could have happened but was more than happy when the reboot sorted it, as the next avenue was to open an expensive support ticket with MS support.