Are you getting a connection in the same script and not terminating ?
If you have a connection open, and the corresponding db2 server process if forced off, the client process does not know of this termination ... Hence, the next time it tries to use the db2 process, it terminates abnormally with sql1224n ...
So, in case you are creating a connection, make sure to terminate it before doing the force applns or before backup ...
If you think this is not your problem, just retry the backup command and it should work now
Cheers
Sathyaram