You know I had something like that happen to me, and for the life of me, I can't rememeber what it was. It could be you are calling close on the data class when the client ends, which will not allow access to anyone else after that. What is you cleanup code for your client when it quits.
I also had it when I modified the db.db file in client code, but that was because I was doing it wrong and corrupting the db.db file.
Mark