If this error causes your entire site to be unavailable, go to wp-admin > Plugins and temporarily disable the WP Staging plugin until we fix the issue.
wp-content/plugins/wp-staging(-pro) with FTP to disable the WP Staging plugin.#0 /var/www/vhosts/144/163415/webspace/siteapps/WordPress-51203/htdocs/wp-content/plugins/permalink-finder/includes/pf-404.php(563): kpg_pf_ErrorHandler(2, 'Undefined varia...', '/var/www/vhosts...', 563)
#1 /var/www/vhosts/144/163415/webspace/siteapps/WordPress-51203/htdocs/wp-content/plugins/permalink-finder/includes/pf-404.php(503): kpg_pf_load_page(1)
#2 /var/www/vhosts/144/163415/webspace/siteapps/WordPress-51203/htdocs/wp-content/plugins/permalink-finder/permalink-finder.php(37): kpg_permalink_fixer()
#3 /var/www/vhosts/144/163415/webspace/siteapps/WordPress-51203/htdocs/wp-includes/class-wp-hook.php(307): kpg_permalink_finder('')
#4 /var/www/vhosts/144/163415/webspace/siteapps/WordPress-51203/htdocs/wp-includes/class-wp-hook.php(331): WP_Hook->apply_filters(NULL, Array)
#5 /var/www/vhosts/144/163415/webspace/siteapps/WordPress-51203/htdocs/wp-includes/plugin.php(476): WP_Hook->do_action(Array)
#6 /var/www/vhosts/144/163415/webspace/siteapps/WordPress-51203/htdocs/wp-includes/template-loader.php(13): do_action('template_redire...')
#7 /var/www/vhosts/144/163415/webspace/siteapps/WordPress-51203/htdocs/wp-blog-header.php(19): require_once('/var/www/vhosts...')
#8 /var/www/vhosts/144/163415/webspace/siteapps/WordPress-51203/htdocs/index.php(17): require('/var/www/vhosts...')
#9 {main}