If someone would know the answer or someone ever experienced this problem, then you would have got an answer already. This has nothing to do with being a subscriber or not. You had a different kind of mail system installed before which still interferes with the current setup.
If you wnat to get paid support, then take a look at the support page on ispconfig.org, you will find the contact information there were you can contact us. The howtoforge forum is only for the ispconfig community and we will not handle any paid support here.