webaccess not working after upgrade to gw18


we just upgraded our single server system from GW 2014 ( where everything was working ) to Groupwise 18.

Everything seems ok, except webaccess.

http connections are refused, which I understand is the way it is supposed to be with GW 18.

However, also https do not work and lead to the following error:

Server Error
403 - Forbidden: Access is denied.
You do not have permission to view this directory or page using the credentials that you supplied.

Any help is very much appreciated.