Hmm, might be some problem with class loading. Maybe JBoss JARs contain some classes which are used by the HttpClient but have different version.
Try to debug it, (in a debugger, like Eclipse, or I guess JRE has some option for debugging class loading) or try clasloader isolation with parent delegation set to false.
Now I feel so stupid....
Solved it. Somebody (maybe even yours truly) fucked up the config of the test and live system.
There was a typo in the URL for the https server.
Now it works.
And I only found it because I wanted to try and change the DNS name to the actual IP and then it hit me...
Damn stupid typos in my URLs!
Thanks anyway, guys & girls!