Yesterday I was not able to login to Wikimedia projects. I got a following message: "No active login attempt is in progress". This happened on Firefox browser on PC. I tried to login on my mobile phone (also FF), and there it worked. Then I decided to remove the cache/cookies at Firefox on my PC. After that everything worked normally.
I don't know what caused the problem, and can every users fix it themself.
Also reported at:
https://nl.wikipedia.org/wiki/Help:Helpdesk#Inloggen
https://en.wikipedia.org/wiki/Wikipedia:Help_desk#Can_not_login
https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)#Log_in_problems