Finally I found the root of the problem. Installing the application on jboss also altered my server.xml, adding a security certificate to the https connection. After removing this entry, the test ran perfectly. But the problem is, that this certificate is intended to be used. Is there a possibility to add this to the test somehow?
edit: Damn. Only a modified test worked. In this modified test, the class I tested successfully was not deployed beforehand but within the test package itself. When I try to test a class, thats already installed on the server, I get a ClassNotFoundException.
edit2: This seems to have to do with the installed application too, since everthing works fine on a clean server where a single bean containing a single method is installed. I'm trying to get some more info tomorrow.