Classic editor shows text white on white on my back end

Hello, classic editor only shows white text, that i can only seen when I mark it. On my front end everything luckely looks fine. already tried to deactivate all other plugins (nothing changed) reinstall the classic editor (nothing changes) In the back end, I also get this message: Fatal error: Uncaught TypeError: array_filter(): Argument #1 ($array) must be of type array, null given in /home/httpd/vhosts/jobins.ch/httpdocs/wp/wp-includes/class-wp-user-query.php:485 Stack trace: #0 /home/httpd/vhosts/jobins.ch/httpdocs/wp/wp-includes/class-wp-user-query.php(485): array_filter(NULL) #1 /home/httpd/vhosts/jobins.ch/httpdocs/wp/wp-includes/class-wp-user-query.php(79): WP_User_Query->prepare_query(Array) #2 /home/httpd/vhosts/jobins.ch/httpdocs/wp/wp-includes/user.php(879): WP_User_Query->__construct(Array) #3 /home/httpd/vhosts/jobins.ch/httpdocs/wp/wp-includes/user.php(1794): get_users(Array) #4 /home/httpd/vhosts/jobins.ch/httpdocs/wp/wp-admin/includes/meta-boxes.php(970): wp_dropdown_users(Array) #5 /home/httpd/vhosts/jobins.ch/httpdocs/wp/wp-admin/includes/template.php(1453): post_author_meta_box(Object(WP_Post), Array) #6 /home/httpd/vhosts/jobins.ch/httpdocs/wp/wp-admin/edit-form-advanced.php(721): do_meta_boxes(Object(WP_Screen), ‘normal’, Object(WP_Post)) #7 /home/httpd/vhosts/jobins.ch/httpdocs/wp/wp-admin/post.php(206): require(‘/home/httpd/vho…’) #8 {main} thrown in /home/httpd/vhosts/jobins.ch/httpdocs/wp/wp-includes/class-wp-user-query.php on line 485 The page I need help with: [ log in to see the link]