I've found the solution:
The above mentioned match rule works like it should. But there seems to be a QBW problem with updating the widgets after editing the match rule:
Everytime I edit the match rule, the widget doesn't react on DBUS events anymore. I have to delete the widget an recreate it again. The new widget is the same as the deleted widget - with the edited match rule.

After this delete/recreate procedure the widget works without problems.

This concerns only editing the match rule - every other setting can be changed without breaking the DBUS event listening.