and as update, the Studio works fine, the frontend seems to work only the main categories (listing profiles, groups etc) but for all "deeper" links I get a spinning cursor and after some timeout a blank page or 503 :(

Logs only show warnings

AH01071: Got error 'PHP message: PHP Warning: Undefined array key 0 in /var/www/vhosts/xyz.com/httpdocs/cache/bx_templ_de_artificer_919122ca265e8463e5ce6c4be2905c89.php on line 2; PHP message: PHP Warning: Trying to access array offset on null in /var/www/vhosts/xyz.com/httpdocs/cache/bx_templ_de_artificer_919122ca265e8463e5ce6c4be2905c89.php on line 2; PHP message: PHP Warning: Undefined array key 0 in /var/www/vhosts/xyz.com/httpdocs/cache/bx_templ_de_artificer_919122ca265e8463e5ce6c4be2905c89.php on line 3; PHP message: PHP Warning: Trying to access array offset on null in /var/www/vhosts/xyz.com/httpdocs/cache/bx_templ_de_artificer_919122ca265e8463e5ce6c4be2905c89.php on line 3; PHP message: PHP Warning: Undefined array key 0 in /var/www/vhosts/xyz.com/httpdocs/cache/bx_templ_de_artificer_919122ca265e8463e5ce6c4be2905c89.php on line 5; PHP message: PHP Warning: Trying to access array offset on null in /var/www/vhosts/xyz.com/httpdocs/cache/bx_templ_de_artificer_919122ca265e8463e5ce6c4be2905c89.php on line 5'

EDIT: here is a real error:

AH01071: Got error 'PHP message: PHP Fatal error: Uncaught Error: Call to a member function addMarkers() on false in /var/www/vhosts/xyz.com/httpdocs/modules/base/general/classes/BxBaseModGeneralMenuViewActions.php:65\nStack trace:\n#0 /var/www/vhosts/xyz.com/httpdocs/inc/classes/BxDolMenu.php(123): BxBaseModGeneralMenuViewActions->addMarkers()\n#1 /var/www/vhosts/xyz.com/httpdocs/template/scripts/BxBaseMenu.php(25): BxDolMenu->__construct()\n#2 /var/www/vhosts/xyz.com/httpdocs/modules/boonex/artificer/data/template/system/scripts/BxTemplMenu.php(17): BxBaseMenu->__construct()\n#3 /var/www/vhosts/xyz.com/httpdocs/template/scripts/BxBaseMenuMoreAuto.php(33): BxTemplMenu->__construct()\n#4 /var/www/vhosts/xyz.com/httpdocs/template/scripts_templ/BxTemplMenuMoreAuto.php(17): BxBaseMenuMoreAuto->__construct()\n#5 /var/www/vhosts/xyz.com/httpdocs/template/scripts/BxBaseMenuCustom.php(18): BxTemplMenuMoreAuto->__construct()\n#6 /var/www/vhost...'