Creating Users doesn't work properly

Hello Everyone,

Just installed nextcloud 11 on a local server with nginx mariadb and php7 on ubuntu 16.04 server

Everything is running fine except creating users and changing the passwords.

Now you can create a user, but a plain password like 12345678 doesn’t work. Only if the password is complex enough the user is created. If you try changing the users password it doesn’t work.

I am not running any encryption on the server. Also all files in /var/www/nextcloud are owned by www-data.

Everything else works uploading video and images, watching them back, opening text files.

Whats is going on?

Edit:

Heres what error my browser gives me: