[TYPO3] Ext->TMailform -> different behaviour on different domains

[Klaus Musset]

Hi everybody,
this is a nut:
I installed the TMailform, works fine, but this happens:
on 1 domain www.behaim.com the form shows
http://www.behaim.com/index.php?id=kontaktformular
on the other it doesn't
http://koerperkalligraphie.de/index.php?id=kontaktformular

I installed it for now for sample in the same typo3 4.0.1 installation 
(I tried it on 4.0.2 -> same outcome), the same page tree, the same page.

www.behaim.com has been routed to the typo directory by my provider
koerperkalligraphie.de has been routed to the typo directory by me

My provider says everything is identical on the server?!?
Does anybody has a clue what it could be?
Cheers and thanx4help
Klaus