Bad Request - Request Too Long - HTTP Error 400. The size of the request headers is too long.
This usually means that you have one or more cookies that have grown too big. Visit https://support.microsoft.com and - assuming the site opens normally - click on the padlock at the left-hand end of the address bar to open the site info pop-up. Click on Cookies to reveal all the cookies that the site has set (when I just did this, there were 140 of them). Then select each cookie folder in turn and click Remove.
Restart the browser, then try visiting the URL you were having trouble with earlier. If you were signed in before, you'll probably have to sign in again.