This could happen if, for instance, you have a one-user license and multiple users are trying to access the database simultaneously. If this happens on a stand-alone computer, could it be that you are using multiple users accounts on it, in quick succession? Or, maybe the database is read-only and has an old user login stuck inside (the current users are kept track of in the database)? You could use the free [link=http://www.alexnolan.net/software/mdb_viewer_plus.htm]MDB Viewer Plus[/link] tool to view the ActiveUsers table in your database file (*.tpdb8) to see if that provides any hints.