Error when using private file system

while trying to setup a field upload to private file system I get an error:

steps to reproduce:

install module. try to set upload destination of a content type field from "public" to "private".

Browser shows
Oops, something went wrong. Check your browser's developer console for more details.

log shows:

LogicException: The database connection is not serializable. This probably means you are serializing an object that has an indirect reference to the database connection. Adjust your code so that is not necessary. Alternatively, look at DependencySerializationTrait as a temporary solution. in Drupal\Core\Database\Connection->__sleep() (Zeile 1920 in /var/www/html/web/core/lib/Drupal/Core/Database/Connection.php).