No login after update: Possible CSRF attack. Connection will be closed

I have Nextcloud installed on my shared web hoster (

When I noticed that my Windows Nextcloud client would not synchronize my files, I tried to log in on the web page.

I got this message:

Nextcloud wird auf Version 17.0.1 aktualisiert

When I clicked on “Aktualisierung starten” I got this error message:

“Possible CSRF attack. Connection will be closed.”

Reloading does not help.

My web hoster (1blu) manages my web application and installs updates automatically.

I don’t have shell access. I could edit the config files via ftp.

What can I do now?

Thank a lot! Any help is appreciated very much.