Nevermind, I managed to fix it. Maybe good to know for others in case they have the same problem: I temporarily deactivated all plugins as described here:
https://wpmayor.com/what-to-do-when-a-wordpress-plugin-causes-your-website-to-crash/
After that I could login again and I removed the plugin. So, I still haven’t encrypted my website but I decided this can wait a while. I need to get over this shock…
