Hi,
If you have already compared the settings of working and non working users and tried deleting and re-importing the user, it could possibly be the following bug
https://tools.cisco.com/bugsearch/bug/CSCue31224/?reffering_site=dumpcr
You may check if this indeed is a user with large mailbox and try implementing the workaround mentioned in the bug and see if it helps.
Manish