I run
EXECUTE SYS.dbms_system.set_ev (42, 45529,10046,12,'');
or
EXECUTE SYS.dbms_system.set_sql_trace_in_session (42, 45529, TRUE);

PL/SQL procedure successfully completed.

and no trace files are generated in ../udump

I am quite sure that I supply correct sid and serila#
Any ideas ?

Thanks