C/ODBC and SQL Linked Servers

I’ve set up a linked server on SQL2000 to our Navision Database with ODBC. The strangething about that is, when I start my querry the first time then it works. When I try to access the tables the second time i get the following error. [OLE/DB provider returned message: [Microsoft Business Solutions ApS][Microsoft Business Solutions-Navision ODBC Driver]Invalid login] Has anyone an idea what to do? any help apreciated. Cheers / axl