Fatal error: Uncaught Error: Call to undefined function is_main_site() in /usr/home/bmserwer1/domains/paniburger.pl/public_html/wp-includes/update.php:1149
Stack trace:
#0 {main}
  thrown in /usr/home/bmserwer1/domains/paniburger.pl/public_html/wp-includes/update.php on line 1149