Fatal error: Uncaught TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback, class RSS_Autoren_Plugin does not have a method "add_autoren_css" in /var/customers/webs/Muhsin/muhsin.de/wp-includes/class-wp-hook.php:324 Stack trace: #0 /var/customers/webs/Muhsin/muhsin.de/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters() #1 /var/customers/webs/Muhsin/muhsin.de/wp-includes/plugin.php(517): WP_Hook->do_action() #2 /var/customers/webs/Muhsin/muhsin.de/wp-includes/general-template.php(3192): do_action() #3 /var/customers/webs/Muhsin/muhsin.de/wp-content/themes/minformatik/header.php(18): wp_head() #4 /var/customers/webs/Muhsin/muhsin.de/wp-includes/template.php(810): require_once('...') #5 /var/customers/webs/Muhsin/muhsin.de/wp-includes/template.php(745): load_template() #6 /var/customers/webs/Muhsin/muhsin.de/wp-includes/general-template.php(48): locate_template() #7 /var/customers/webs/Muhsin/muhsin.de/wp-content/themes/minformatik/index.php(15): get_header() #8 /var/customers/webs/Muhsin/muhsin.de/wp-includes/template-loader.php(106): include('...') #9 /var/customers/webs/Muhsin/muhsin.de/wp-blog-header.php(19): require_once('...') #10 /var/customers/webs/Muhsin/muhsin.de/index.php(17): require('...') #11 {main} thrown in /var/customers/webs/Muhsin/muhsin.de/wp-includes/class-wp-hook.php on line 324