Also, let me mention that the problem only occurs in child theme. If I change to parent theme, everything works normally.
Unfortunately the same problem again. With the debugging mode enabled, the message I get is:
Notice: The add_submenu_page function was incorrectly called. The seventh parameter sent to add_submenu_page () must be an integer that represents the position of the menu. Please see the Debugging in WordPress page for more information. (This message was added to ed. 5.3.0 ) in /var/www/vhosts/efthimistzimos.sites.sch.gr/httpdocs/wp-includes/functions.php on line 5831
DEPRECATED: Hook contextual_Help is obsolete by version 3.3.0 onwards! Use Get_Current_screen ()-> add_Help_tab (), get_current_screen ()-> remove_help_tab () instead. in /var/www/vhosts/efthimistzimos.sites.sch.gr/httpdocs/wp-includes/functions.php on Line 5754
(google translated from Greek)
