ora-00030 user session id does not existwhen killing a session with ALTER SYSTEM KILL SESSION
In v$session, the session was visible as ACTIVE, so why is the user session not existent?
The solution: Keep in mind, that you are connected to the correct node when using a database RAC. In my case, killing the session was only successful on the instance of that session. Look the node up using this statement:
select s.sid, s.serial#, i.host_name, s.username from
gv$session s join
gv$instance i on (i.inst_id=s.inst_id)
username is not null;