Setting up nextcloudpi on a public IP address fails

I’m trying to set up a nextcloudpi instance on a fully routed public IP address.

What is working: when I set up the instance on a private IP with DHCP all working fine

What is not working: when I set up the instance, set dhcpcd.conf to public static IP and change nc-static-IP to this static IP … result on browser is always “Forbidden You don’t have permission to access this resource.”

I had this effect on former installations of NC when I tried to migrate an instance firstly set up with DHCP to a later static public IP. Installing NC from scratch on an Ubuntu server allows you to set up static public IP during set up, then all is fine - but on nextcloudpi all comes preinstalled.

I guess the problem is somewhere in the Apache setup but while I’m good on networking, I’m not on Apache …

Any idea much appreciated !

Best Regards