Let me first say that it appears to be working now as before, without me doing anything – so perhaps it was a caching issue?
Anyway, in case you are still interested: when I tried to login to get to the backend I did never see the login page, but was forwarded to the main page of the domain. Only after I renamed the folder better-wp-security in the plugins folder to better-wp-security.disabled, I was able to login.
Even if it works now, I would still be interested in possible causes.
Thanks,
Alexander
