Hapewe

I had to activate the debug mode, which stated the “Fatal error: Cannot redeclare url_shorten() (previously declared in /home/www/xxx/yyy/wp-inlcudes/formatting.php:5770)in /home/www/xxx/yyy/wp-admin/includes/misc.php on line 228”
A quick search gave me the advice to manually (by ftp) upload a fresh wp version (except the content folder).
This did the trick. Website works again, backend can be reached again.