Mail App installation fails

During installation of th mail app I get this messeges in logfile:

Error: Call to a member function getContainer() on null

/var/www/nextcloud/lib/private/AppFramework/Bootstrap/Coordinator.php - line 143:

OC\AppFramework\Bootstrap\RegistrationContext->delegateContainerRegistrations({ mail: OCA\ ... }})

/var/www/nextcloud/lib/private/AppFramework/Bootstrap/Coordinator.php - line 86:

OC\AppFramework\Bootstrap\Coordinator->registerApps([ "mail"])

/var/www/nextcloud/lib/private/Installer.php - line 146:

OC\AppFramework\Bootstrap\Coordinator->runLazyRegistration("mail")

/var/www/nextcloud/apps/settings/lib/Controller/AppSettingsController.php - line 448:

OC\Installer->installApp("mail")

/var/www/nextcloud/lib/private/AppFramework/Http/Dispatcher.php - line 218:

OCA\Settings\Controller\AppSettingsController->enableApps([ "mail"], [])

/var/www/nextcloud/lib/private/AppFramework/Http/Dispatcher.php - line 127:

OC\AppFramework\Http\Dispatcher->executeController(OCA\Settings ... {}, "enableApps")

/var/www/nextcloud/lib/private/AppFramework/App.php - line 157:

OC\AppFramework\Http\Dispatcher->dispatch(OCA\Settings ... {}, "enableApps")

/var/www/nextcloud/lib/private/Route/Router.php - line 302:

OC\AppFramework\App::main("OCA\\Settin ... r", "enableApps", OC\AppFramew ... {}, { _route: "s ... "})

/var/www/nextcloud/lib/base.php - line 993:

OC\Route\Router->match("/settings/apps/enable")

/var/www/nextcloud/index.php - line 37:

OC::handleRequest()

vor 5 Minuten
Error PHP Error: Undefined index: comments at /var/www/nextcloud/lib/private/AppFramework/Bootstrap/RegistrationContext.php#404

/var/www/nextcloud/lib/private/AppFramework/Bootstrap/RegistrationContext.php - line 404:

OC\Log\ErrorHandler::onError(8, "Undefined index: comments", "/var/www/ne ... p", 404, { apps: { ma ... }})

/var/www/nextcloud/lib/private/AppFramework/Bootstrap/Coordinator.php - line 143:

OC\AppFramework\Bootstrap\RegistrationContext->delegateContainerRegistrations({ mail: OCA\ ... }})

/var/www/nextcloud/lib/private/AppFramework/Bootstrap/Coordinator.php - line 86:

OC\AppFramework\Bootstrap\Coordinator->registerApps([ "mail"])

/var/www/nextcloud/lib/private/Installer.php - line 146:

OC\AppFramework\Bootstrap\Coordinator->runLazyRegistration("mail")

/var/www/nextcloud/apps/settings/lib/Controller/AppSettingsController.php - line 448:

OC\Installer->installApp("mail")

/var/www/nextcloud/lib/private/AppFramework/Http/Dispatcher.php - line 218:

OCA\Settings\Controller\AppSettingsController->enableApps([ "mail"], [])

/var/www/nextcloud/lib/private/AppFramework/Http/Dispatcher.php - line 127:

OC\AppFramework\Http\Dispatcher->executeController(OCA\Settings ... {}, "enableApps")

/var/www/nextcloud/lib/private/AppFramework/App.php - line 157:

OC\AppFramework\Http\Dispatcher->dispatch(OCA\Settings ... {}, "enableApps")

/var/www/nextcloud/lib/private/Route/Router.php - line 302:

OC\AppFramework\App::main("OCA\\Settin ... r", "enableApps", OC\AppFramew ... {}, { _route: "s ... "})

/var/www/nextcloud/lib/base.php - line 993:

OC\Route\Router->match("/settings/apps/enable")

/var/www/nextcloud/index.php - line 37:

OC::handleRequest()

What ist to do??