Yes, for sure it's not a problem of you and your ISPconfig software. I know SuSE sometimes does strages things. :-( But we always do SuSE Linux Enterprise Server in our company and had never had such strage behaviours - so I decided to use SuSE in the private sector too. ;-)
But now I switched to a Ubuntu 6.10 vServer. ISP installed perfect there. I already have other questions, but I think I should post them in a separate thread.
Thanks for all answers!