Steps to replicate the issue (include links if applicable):

• Edit Wikipedia and suddenly get logged out
• Try to log in again

What happens?:
With a firefox browser, I'm trying to log back in after I got logged out. It frequently happens that one suddenly gets logged out and that problem is bad enough, other users have noted this as well. But now I also can't log back in: when I enter the correct password it displays a red box with:

There seems to be a problem with your login session; this action has been canceled as a precaution against session hijacking. Please resubmit the form. You may receive this message if you are blocking cookies.

I already tried logging in an incognito session and deleting site cache and cookies (left of the url in url bar in Firefox).

This task seems related: T263617

What should have happened instead?:

• It should not log out users
• If it does, it should allow logging in again

Software version (skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):
English-language Wikipedia
This never happened before