Hi @Ireas, please follow https://www.mediawiki.org/wiki/Manual:How_to_debug/Login_problems and report back here. Thanks!

The error seems erratic to me..

• std. machine: i tried to login with commons (work now), yesterday not. login to mediawiki failed with (There seems to be a problem with your login session ....)

• Test different machine/browser
  • 1. wikimedia works (login/logout), wikisource did not (There seems to be a problem with your login session)
  • 2. login wikimedia seems to work. after login i got OAuth page with correct name, after pressing "allow" i was ask to create a new account. (I stopped here because i was confused, why do i need a new account when i was successfully identified).

• Extensions:
  • i use noscript by default, was no problem so far

• other wikis
  • I did not notice problems with other wikis like en. etc.

I just noticed that i was loged out from the wiki without my intervention
while i was writing this mail on an other window.

After i had a unexplained logout , now i can not login on my std. machine nor on a second both reply with.
Es gab ein Problem bei der Übertragung deiner Benutzerdaten. Diese Aktion wurde daher sicherheitshalber abgebrochen, um eine falsche Zuordnung deiner Änderungen zu einem anderen Benutzer zu verhindern. Bitte sende das Formular erneut ab.

After several attempts i tried with javascript enabled and i could login again (causality ??)

See the unanswered bullet points / questions on https://www.mediawiki.org/wiki/Manual:How_to_debug/Login_problems