Hi Nima,
From the logs, I'd wonder whether the 'signal 11' is coming from AOLserver or from Postgres. There was a similar report here earlier which was solved by changing libraries:
https://openacs.org/forums/message-view?message_id=318659
If not, I'd look in the PG logs and see if it's reporting anything. There are a bunch of google hits for "fatal signal 11 postgresql", but I didn't investigate them much further.
