Cannot log in to panel after 2.4.1 update (…but no problems with 2.3.2)

I tried to update one of my Kirby installations from 2.1 to the most recent version 2.4.1, which worked fine on the front-end. I was not able to access the panel, though, getting redirected to the /panel/login/ page with each attempt and with any user.

Changing the PHP version from 5.6.x to 7.0.x did not help, neither did to HTTPS, nor the implementation of a session fingerprint via config file, as described here – all of which without any clues from the error log.

What worked, oddly: switching to the second most recent release, 2.3.2.

Is this a common issue with Kirby CMS 2.4.x? What might be the cause?

Thanks in advance for any kind of help, and
Kind regards,

Dominik

Did you check all points at Installation issues, Basic error hunting & prevention and Debugging from the docs?
Have you checked the permissions and the groups of your directories and files on your websever after updating Kirby?

Good luck!

Are you using any redirects in your .htaccess?
What is in your config.php?

I could solve almost all my login problems in the past by deleting the site/accounts/.login file. Kirby will regenerate it automatically. But be aware that all currently logged in users might get logged out.

Maybe setting the right permissions for this file without deleting it could also solve the issue. You should try this first.