WP installation broken

Hey guys,

until yesterday i was using WP 4.3 without trouble on my website. Now all I’m getting is a blank site and a redirection to https://www.mywebsite.de/defaultsite, which is also blank. If i want to access any other site which i know is existing (www.mywebsite.de/knownsite) is also a blank site. In my desperation i installed my backup from last night of the server and that didn’t change anything. I replaced the theme by a new one and renamed the old one and i disabled all plugins by renaming the plugins-folder and creating a new blank one. Nothing changed and now I’m close to install a fresh wordpress… Is anybody out there with an idea which rescues my page? Thank you!