I've had the exact same error in 5.1 when searching the system tree with a specific epo user resulted in no results.
Had to delete the user and recreate the account to fix this...
Message was edited by: jelmerm on 11/20/13 5:19:52 AM CST
I was hoping to avoid deleting the user, as last time I deleted a user, it also deleted everything they had ever created: queries, reports, policies etc.
I have ended up rolling back to ePO 5.0 due to the amount of other problems that were occuring so the problem is sort of resolved