When Offline Vault is enabled it is possible that an issue could occur with the offline databases that stops Offline Vault downloading correctly. In a Client Trace set to maximum logging this will appear similar to the following results below.  The database number etc may appear differently and be logged varying amounts of times for the same item.  
Note: One entry per item is by design and can safely be ignored, this TechNote is only applicable if the below appears multiple times for the same item with different database numbers.

1/09/2007 09:00:34.637[2592]: ~PVHelper::SetPVIDOnItem: 0x0
1/09/2007 09:00:34.637[2592]:         Processing item: Updated: Test Email
1/09/2007 09:00:34.638[2592]:             (resetting invalid database: 5)
1/09/2007 09:00:34.647[2592]: ~CPVTrawler::ProcessItem: 0x0

Solution

1: Rename cached user profile that is experiencing the issue located in "Documents and Settings".  In addition if using a roaming profile this will also require renaming.

2: Re-Configure user profile and test Offline Vault to confirm issue is resolved.