I reinstalled the regional nagios with Nagios3 and it works now.
As suggested by Steve I'm also trying the nagios checker plugin
instead of the email notification but I still have to configure things properly. At the moment firefox makes some noise every ~30 seconds and there is also a visual alert on the bottom right corner of the firefox window with the number of services in critical state which expands to show the services when the cursor points at it. Really nice. :)