Running piwigo on nextcloudpi fails

Piwigo installation on a “fresh” nextcloudpi device fails.
The installation follows this documentation.

Main problem is, that when I request the address https://radius.ddnss.de/piwigo the webserver redirects the request to https://radius.ddnss.de, that is the root URI of nextcloud server.

My question is, what should be considered for piwigo?