I tried to figure out what was wrong again.
What amazes me now is when I call the script address http://server/enrol/plugin/ipn.php directly in the browser the following happens:
At the test server: blank page
For the production server with 3.3.4+: blank page
For the production server with 3.4.2: Direct redirection to http://server
Does anyone have any idea how this could happen? Where can such settings be made? Before the update this forwarding was not active...