OK, please read http://forum.avast.com/index.php?topic=82941.msg680947#msg680947 for the difference between http://:82/Avast/Default.aspx and http://:82/Avast/Service.aspx.

Now to your actual problem, there is a known bug related to custom port (for Default website selection). But there is also a workaround:

  • Let’s assume 82 is a port currently selected for Default website
  • Go to http://:82/Avast/Service.aspx (unsecure access, but should work) for access to Administration Console
  • Go to Admin->Settings->Advanced mode
  • Change Console address setting in Server core from /Avast/Service.aspx to :82/Avast/Service.aspx
  • Restart the avast! Administration Console windows service

Now you should be able to access the secure http://:82/Avast/ or http://:82/Avast/Default.aspx normally. Please let me know if described steps solved your problem.