Backend not available

[Kernig]

Hello,

we have occasional problems to reach our Prestashop backend. 
When calling the backend, we are shown a white page. We have noticed that empty index.php files are created repeatedly. If these are removed, we can access the backend as usual. 
In the log we get the following error.
PHP Fatal error:  Uncaught Symfony\\Component\\DependencyInjection\\Exception\\InvalidArgumentException: Expected to find class "PrestaShopBundle\\Controller\\Admin\\Sell\\index" in file "URLPLACEHOLDER/src/PrestaShopBundle/Controller/Admin/Sell/index.php" while importing services from resource "URLPLACEHOLDER/app/../src/PrestaShopBundle/Controller/*", but it was not found! Check the namespace prefix used with the resource. in URLPLACEHOLDER/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/Loader/FileLoader.php:157
Stack trace:
#0 URLPLACEHOLDER/vendor/symfony/symfony/src/Symfony/Component/DependencyInjection/Loader/FileLoader.php(57): Symfony\\Component\\DependencyInjection\\Loader\\FileLoader->findClasses('PrestaShopBundl...', '/var/www/vhosts...', '/var/www/vhosts...')
#1 URLPLACEHOLDER/vendor/symfony/symfony/src/Symfony/Component/Config/Loader/FileLoader.php on line 168
We also activated debug mode and received the error message. (See image)
We are currently using Prestashop version 1.7.7.3 with the Leoneons theme.