The version 5.6.11 have a fatal error :
Fatal error: Uncaught TypeError: Return value of RWMB_Update_Option::get_api_key() must be of the type string, null returned in /home/clients/250d932eb38e3315e188024ff2c4dec1/web/wp-content/plugins/meta-box/inc/update/option.php:28 Stack trace: #0 /home/clients/250d932eb38e3315e188024ff2c4dec1/web/wp-content/plugins/meta-box/inc/update/checker.php(135): RWMB_Update_Option->get_api_key() #1 /home/clients/250d932eb38e3315e188024ff2c4dec1/web/wp-content/plugins/meta-box/inc/update/checker.php(83): RWMB_Update_Checker->request('plugins') #2 /home/clients/250d932eb38e3315e188024ff2c4dec1/web/wp-includes/class-wp-hook.php(310): RWMB_Update_Checker->check_updates(Object(stdClass)) #3 /home/clients/250d932eb38e3315e188024ff2c4dec1/web/wp-includes/plugin.php(205): WP_Hook->apply_filters(Object(stdClass), Array) #4 /home/clients/250d932eb38e3315e188024ff2c4dec1/web/wp-includes/option.php(2018): apply_filters('pre_set_site_tr...', Object(stdClass), 'update_plugins') #5 /home/clients/250d932eb38e3315e188024ff2c4dec1/web/wp-includes/u in /home/clients/250d932eb38e3315e188024ff2c4dec1/web/wp-content/plugins/meta-box/inc/update/option.php on line 28