Hi pschulz,
First of all, thanks very much for purchasing iRedAdmin-Pro to support iRedMail project.
The reason why we ask to use a FRESH OS is avoiding issues like yours, too many things we cannot know on your server. But with a FRESH OS, everything is under control.
So, my suggestion is, reinstall your OS on Xen with minimal packages, then try iRedMail.
P.S. If you're using Windows workstation and connect to server with SecureCRT as ssh client, there may be issue while configuring iRedMail. Please try other SSH client instead. e.g. Putty.