I keep getting the following error when I sync with one particular user.

"The GroupWise user login has changed since the last synchronization. This could result in data loss in GroupWise or on the device. Click cancel to stop the synchronization or click continue to proceed with the synchronization."

The only info google turned up about this error message is on a palm forum. I just spent 3 1/2 hours on the phone with tech support, and they ran out of suggestions. It is definitely a problem with the user's GW mailbox, but content and structure checks turned up nothing, and rebuilding the database did nothing. Anyone have any other suggestions?

Have you tried removing the user and re-adding them to the BES?
Also you may want to contact RIM directly and have them direct you to the DBClean utility to clean users account;