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.

Is this a known problem?

Well, the problem with FireFox is known to several users here.

I personally don't have the login problem with my FireFox, but then I use a very outdated version (56.0.1) and also access CPlusPlus via the legacy version of the site.

I did find this about the login problem:
https://cplusplus.com/forum/lounge/284671/#msg1233786

Peter87 wrote:
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.

Accessing the legacy version:

https://legacy.cplusplus.com/

The new and 'enhanced' version that is default now is for me more than a bit off-putting.

I just now tested logging out and back in using the legacy site's interface. Nary a problem, just add the appropriate account info and click log-in.

FYI, the 'new and enhanced' current version served up as default is shown as still being "Beta" when logging in via the legacy site. ::shrug::