I got a workaround for my problem:
After updaten WordPress 5.6 the problem with submenu and mobile menu appears. So i was downgrading WordPress to 5.5.3 and now it is working.
Bevor downgrading make sure to change theme to standard wp theme and deactivate all plugins. Then overrite wp-includes and wp-admin with old version of wordpress.
In my case sidebars, menu and static landingpage have to be set again!
@Support hope there is an update soon so i can update wordpress to 5.6.
PS: Plugin Ultimate Addons for Visual Composer can’t be activate becaus of fatal php error (wp-content/plugins/Ultimate_VC_Addons/Ultimate_VC_Addons.php on line 28)