ORA-12519, TNS:no appropriate service handler found

I'm using Enterprise Manager to create a standby database. The standby database is created on a separate server but when I try to "select existing standby database"  I get the following error:

Listener refused the connection with the following error: ORA-12519, TNS:no appropriate service handler found The Connect Descriptor was (description=(address=(host=MA0183.global.ctb)(protocol=tcp)(port=1521))(connect_data=(service_name=CONVDR)(instance_name=CONVDR)(UR=A)))

If I login on the remote server and set oracle_sid=convdr and I can connect to the database using sqlplus.

Any clues on how to fix this problem.

Thanks,