I already figured it out. As you implied that too many trace records in trace tables and my system tablespace is 100% full.
I truncated those trace tables and it's OK now.
Another question pop-up in my mind:
How does PL/SQL Developer take care of individual session for tracing and profiling?
If a group of developers working in the same schema traces or profiles their own procedures, can PL/SQL Developer return ONLY their own trace or profile records?
Thanks,