Try this sql:
select trigger_name from dba_triggers where TRIGGERING_EVENT = ('ERROR')
If you have a trigger with TRIGGERING_EVENT = 'ERROR', is big the possibility of hang. Try disable it and do a test..
If this don't work, see if you have any of this types of trigger too:
'LOGON',
'CREATE OR ALTER',
'CREATE',
'ALTER',
'ALTER OR RENAME',
'LOGON',
'LOGOFF',
'TRUNCATE'
select trigger_name from dba_triggers where TRIGGERING_EVENT in ('LOGON','CREATE OR ALTER','CREATE','ALTER','ALTER OR RENAME','LOGON','LOGOFF','TRUNCATE')
Try disable it too and repeat the test..
Good lucky