Firstly avast isn't a firewall and it doesn't block ports, just scans http traffic and alerts you if there is a problem. So perhaps this is some form of conflict with another application.
The network shield only monitors certain ports used for the likes of Sasser exploits and it too doesn't block if no infection is found, in which case it would alarm.
Can you give more information on how it blocks, e.g. what were you doing ?
What is your firewall ?
Reading this link
http://www.databasejournal.com/features/oracle/article.php/3332361 is it not possible to use/try a different port and doses that work ?
