TNS listener error even when database is up

[dereliction]

Hello,
I'm trying to execute some non expensive queries against an Oracle 10g database.I get a listener error, even though the database is mounted and open.

Any suggestions on why this happens.

Previously, listener errors always have resulted from database being in non mount state etc, and so starting the database had solved the problems.

edit: the listener is fine as well when I see its status.
Thanks.

edit: this is the error code form the logs:

ORA-12519, TNS:no appropriate service handler found