> Size of a request header field exceeds server limit. The header field most notorious for growing and growing is the cookies field. Since private windows do not re-use the cookies from regular windows, your private window test supports this theory. I suggest trying to clear your cookies for that site. Here's how: * While viewing a page on the site, click the lock icon at the left end of the address bar. * After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. * Go ahead and click that. Firefox will ask you to confirm; go ahead and confirm. Then try reloading the page and logging in again. Does that help?