If you don't require SSL for your newsgroup, then change the avast Mail Shield, Expert Settings, SSL Accounts setting for that NNTP account to Encryption 'None' should resolves it (as you found).
I don't know what your NNTP settings are for your giganews (or email program) were, if it is using a different port (not 119) or if you have SSL enabled in the account settings ?
If they weren't the standard NNTP port 119 then avast may have thought it required SSL.