After installing Exchange 2010 SP2 Update-Rollup 3, you might see the following error pop-up in the event logs: Unhandled Exception "User setting ‘PreferredSite’ is not available. "
Although at first not much information was available, Greg Taylor already said on the 13th of June that the error is known and they were looking into it.
Apparently, Scott Schnoll also mentioned this in his presentation @ TechEd, stating the following:
It’s good to see that the errors (although presenting them as critical), do no harm. Unfortunately, the more mailboxes you have, the more errors you’ll have in your event log. From what I can see, there might even be an error every few seconds; therefore possibly flooding your event log.
Microsoft will make an Interim Update available. However; at time of writing no fix was available yet.