I came across this error while executing csalter.plb to change db characterset. This was in one of test environment
Alter the database character set...
ERROR at line 1:
ORA-12721: operation cannot execute when other sessions are active
ORA-06512: at "SYS.DBMS_SQL", line 1199
ORA-06512: at "SYS.CSM$RUNDML", line 6
ORA-06512: at line 22
I had restarted database in restrict mode and was the only user connected. Since this was dev environment, I did a small hack to fix this
SQL> update csmig.CSM$PARAMETERS set value='FALSE' where name='CSALTER_FINISHED';
Don’t try this in prod environment. I would recommend doing clean database shutdown and ensuring all jobs are disabled along with setting aq_tm_processes=0 (In case you are using AQ).