Exchange 2007 EMC crashes when adding mailbox permissions

Exchange 2007 sp3 roll up 12.

Whenever I try to add a user to a mailbox the console crashes when searching the list of available users.

Increased diagnostics for MSExchangeADACccess and MangementConsole. When crash takes place I get a 2128 event logged Store.Exe (PID=2276) witha GUID that can't be found on our domain controller. I did a search for the GUID and found nothing.

I can still add permissions through the Shell.

I have no idea how to fix this.

Please help.

Thanks

MJK