Accessing K2 web applications like Designer, K2 Management, Workspace, Tasks etc in some cases presents a "HTTP Error 400. The size of the request headers is too long (Bad Request)" error.
You are unable to access K2 sites.
This is most probably caused by having too many cookies in the header during a request to K2 Trust. It's common when using the same browser session to access many sites that each start adding to the cookies in the browser session.
Clearing the browser cache will fix the issue.