I am getting the following error message in the Sync Log when I try to use Salesforce for Outlook to sync contacts:  "Object reference not set to an instance of an object".  No data is being synced.  This error seemed to pop up randomely.  I was not experiencing this issue earlier today and I have no idea what could have caused it.  Here's the full error message:

2012-12-06 22:59:48,396 ERROR [Contact] ***********  Sync Engine  ***********  Error Sfdc.Outlook.OLSessionException: Object reference not set to an instance of an object. ---> System.NullReferenceException: Object reference not set to an instance of an object.
   at Sfdc.Outlook.categories.Deserialize(String xml)
   at Sfdc.Outlook.OLSession.GetCategoriesFromFolder(IRDOFolder calendarFolder)
   at Sfdc.Outlook.OLSession.GetCategoriesFromStoreOrSession(OLRecordId storeId)
   --- End of inner exception stack trace ---
   at Sfdc.Outlook.OLSession.TranslateException(Exception ex)
   at Sfdc.Outlook.OLSession.GetCategoriesFromStoreOrSession(OLRecordId storeId)
   at Sfdc.Synchronization.CategoryManager.VerifyCategoryState(IOLSession outlookSession, SfdcOlEntityType syncEntityType, IList`1 syncFolders)
   at Sfdc.Synchronization.SyncEngine.CheckIfReadyForSync()
   at Sfdc.Synchronization.ContactSyncEngine.CheckIfReadyForSync()
   at Sfdc.Synchronization.SyncEngine.Start()

Any thoughts/suggestions would be greatly appreciated.

Thanks!