Error 400
Your browser sent a request that this server could not understand.
Size of a request header field exceeds server limit.

That means that your browser is trying to send too much at once. Probably, it’s size of the cookies. The easy way to fix it – delete cookies. Of course, you lose all info in the cookies, but at least you can access site again.

If you develop/manage web site and your customers complain of this error message – there are two things to do:

• Review cookies and limit the size of cookies your application stores in the browser.
• Increase allowed request size in web server configuration

To increase request size in Apache, add directive LimitRequestFieldSize to the configuration file. Default value is 8190 bytes. To increase it 2 times add
LimitRequestFieldSize 16380
Apache restart is required.