blank page after logout or other changes in dashboard

Hi there,
I altered the Blankslate Theme for my purposes and now I’ve got a problem:

– When I logout the browser shows a blank page
– When I try to login it tells me I need to activate the Cookies
– The Login only works when I change the theme’s name fia FTP (as soon as I am logged in I can change it back)

It also displays blank pages when I activate/deactive a plugin or change the settings in the dashboard.

I inserted this code in the index.php in the blankslate folder to find the problem:

<?php ini_set('display_errors','1'); ini_set('display_startup_errors','1'); error_reporting (E_ALL);include('index.php'); ?>

and it tells me the following:
” Notice: Constant WP_USE_THEMES already defined in /usr/www/users/piadin/index.php on line 14 “

I did not change anything in the index.php in the parent directory, neither did I use the variable WP_USE_THEMES anywhere else (At last not consciously…).

What I already tried:
– I deactivated the Plugins one by one
– I changed the name of the Plugin Folder
It didn’t change a thing, so I think the problem lies within my changes in the PHP Documents in the Theme’s folder.

Link to the Webpage: https://piadinabar.de/

Any help is kindly appreciated!