The official Office for Mac blog has published a post detailing an issue that many users have had regarding corrupt databases after updating to Service Pack 2, as well as providing information on how to fix the issue. While the update was meant to fix bugs and improve performance, this is clearly something that got through the net.
The Office 2011 SP2 release improves database resiliency and detects inconsistencies that went unnoticed prior to this release; however some users have experienced issues related to corrupted databases after the latest updates. We apologize to users who were affected by this issue. Users who successfully updated report very noticeable improvements including performance and a host of other fixes.
If you haven’t yet installed SP2, then you should follow the information on this page in order to prepare the database for the upgrade. If you have installed it and are receiving a ‘Please upgrade the Office database’ message, then you should follow the instructions listed here.