403 errors on all websites

Hello all,

I just started getting the following error on all http-sites I visit:

"HTTP Error 403 (Forbidden): The server refused to fulfill the request."

I don't have a problem accessing sites that start with https://.

The problem exists in Chrome (version 21.0.1180.57) and in Safari (5.1.7) where I just get a blank page.

I've found numerous solutions pointing out firewall software but I don't have any installed. There have also been suggestions that a change in proxy settings could help. But all options related to proxy settings are unchecked, checking the automatic one didn't help.

I've been searching for solutions (pretty hard with only access to https-sites) for hours now and have no clue what to do next. I hope someone here can offer some advice?

Thank you!