I can't log to any page using Lynx browser, what I see is "403 Forbidden" message everywhere. I think it has sth to do with User-Agent field, I tried both "Lynx..." and "L_y_n_x...", but also "Opera...", "Mozilla..." etc., but that doesn't help. It could also be related to cookies, but I accept all cookies the browser asks me. Who knows?

Well, no one here will know, when you don't provide any useful details. What sites have you tried? What is the exact error message? Version/distro of Linux? What kind of network connection (home/office/firewalled/etc)???

There are many kinds of 403 errors, also:
http://en.wikipedia.org/wiki/HTTP_403

some have to do with access forbidden, some with SSL, some with user authentication. Provide some details, and perhaps we can help.

Ok, so what I tried was:
1. our school group forum based on phpbb script, where I get "403 Forbidden You don't have permission to access this document."
2. gmail, where after entering my login and password I can only see a very long string that says I will be redirected in 3 seconds, but nothing happens, and when I press this link I get "403 bad request"
3. here: http://www.autohotkey.com/forum/login.php I can't even type my credentials, because I get "Not Acceptable. An appropriate representation of the requested resource /forum/login.php could not be found on this server. Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request"

However, I managed to log into this forum page, the same as keepassx forum, so apparently there are some places that accept my configuration.
I use Slackware 13.1 x64 and I don't use any firewalls.