This could be a sign of database corruption. For Exchange accounts, rather than rebuilding, create a new Identity and let Entourage sync back up.

If you do have info in the folders “On My Computer”, then select work off line. Delete the Exchange account THEN do a rebuild. After the rebuild add the account back.

Make a duplicate in the Finder first as additional backup. A rebuild does make a duplicate, but sometimes in the process the original and backup get merged resulting in total disaster. It’s rare but it happens.

Other steps to try:

• Try repairing Disk Permissions: Open Disk Utility (/Applications/Utilities), select your hard disk volume in the left pane, click the First Aid tab, then click Repair Disk Permissions.
• Re-install Office from original media then updating can resolve issues with unexpected quits. Use “Remove Office” first. Be sure to see on this link the files that are not removed using Remove Office in Office 2008 and will need to removed manually.
• Delete .plist files: Unexpected quits — especially those that occur after a major Mac OS X update — can be solved by deleting corrupt .plist files. Preferences can be found in your User’s Library/Preferences folder:
  • com.microsoft.OfficeNotifications.plist
  • com.microsoft.DatabaseDaemon.plist
  • com.microsoft.Entourage.plist
  • Microsoft folder - com.apple.LaunchServices.plist
• Caches: In some cases, unexpected quits an other issues can be resolved by deleting specific system caches.  Try deleting files that start with “com.apple.LaunchServices” from the /Library/Caches folder.
• A corrupted font cache may be causing Unexpected Quits. The font caches are located in User>Library>Preferences>Microsoft>Office Font Cache.