Both on my laptop and my tablet I have tried to log in, using Firefox. In both cases I see the login dialog flashing by and then a page returns with a '0' in the upper-left corner.
When using Chrome I can login without problems.
I have deleted the cache and the cookies but this did not help.
I personally don't have the login problem with my FireFox, but then I use a very outdated version (56.0.1) and also access CPlusPlus via the legacy version of the site.
As a temporary workaround you can hold down Ctrl while pressing the "log in" button. That'll give access to the login form. The "0" page will open in another tab which you can just close or ignore.