1. thanks for all your responses
2. I just went ahead and DIDN'T install RoundCube over the ISPconfig packet like I used to do before....
I just went ahead and did this HowTo from Hans ... it's a little buggy ... but anyway ... in the end I got it running .... on port 80 .... but anyway .. I think that's actually better than running it on 81 .. because imagine if somebody is working behind a http proxy server and couldn't open the port 81 ... so in the end I think it's the best solve. ...
and my folders looked like this when I tested it the first time:
So that's what I wnat to - and now I have it ;-)