MW 1.35.3, PHP 7.4.27.
The class MobileFrontend\Features\LoggedInUserMode does not implement setEnabled() method declared in MobileFrontend\Features\IUserMode, which causes
[error] 1417#1417: *537 FastCGI sent in stderr: "PHP message: PHP Fatal error: Class MobileFrontend\Features\LoggedInUserMode contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (MobileFrontend\Features\IUserMode::setEnabled) in /var/www/wiki/w/extensions/MobileFrontend/includes/Features/LoggedInUserMode.php on line 20"