Can you give any examples of the forums ?
Do you have to logon before access is granted or does it use cookies ?
What is your browser and your firewall ?
If ZA is it the Pro version ?
avast doesn't block (it isn't a firewall) rather it scans content and if found will sound an alert pop-up an interactive screen, you decide the action and not avast. How do you download these ebooks and do you use a download manager ?