Fatal error: Uncaught TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback, function “_wp_block_theme_register_classic_sidebars” not found or invalid function name in /homepages/46/d4296856183/htdocs/wordpress/wp-includes/class-wp-hook.php:308 Stack trace: #0 /homepages/46/d4296856183/htdocs/wordpress/wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters(”, Array) #1 /homepages/46/d4296856183/htdocs/wordpress/wp-includes/plugin.php(517): WP_Hook->do_action(Array) #2 /homepages/46/d4296856183/htdocs/wordpress/wp-includes/widgets.php(1854): do_action(‘widgets_init’) #3 /homepages/46/d4296856183/htdocs/wordpress/wp-includes/class-wp-hook.php(308): wp_widgets_init(”) #4 /homepages/46/d4296856183/htdocs/wordpress/wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters(NULL, Array) #5 /homepages/46/d4296856183/htdocs/wordpress/wp-includes/plugin.php(517): WP_Hook->do_action(Array) #6 /homepages/46/d4296856183/htdocs/wordpress/wp-settings.php(623): do_action(‘init’) #7 /homepages/46/d4296856183/htdocs/wordpress/wp-config.php(76): require_once(‘/homepages/46/d…’) #8 /homepages/46/d4296856183/htdocs/wordpress/wp-load.php(50): require_once(‘/homepages/46/d…’) #9 /homepages/46/d4296856183/htdocs/wordpress/wp-blog-header.php(13): require_once(‘/homepages/46/d…’) #10 /homepages/46/d4296856183/htdocs/wordpress/index.php(17): require(‘/homepages/46/d…’) #11 {main} thrown in /homepages/46/d4296856183/htdocs/wordpress/wp-includes/class-wp-hook.php on line 308
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disable wp code box or any snippet plugin first. then see if any particular snippet is causing the issue. latest wp changed some stuff, my snippet for max characters for excerpts was causing this.