infcloud PHP auth module not working

[pkoevesdi]

I try to setup the "Special setup" as described here: https://github.com/Unrud/RadicaleInfCloud/tree/master/radicale_infcloud/web. But I can't get it to work.
My working "Standard setup" is reachable under https://carddav.example.com/.web/infcloud/.
If I visit https://carddav.example.com/.web/infcloud/auth/, I get The requested resource could not be found..
If I visit https://carddav.example.com/.web/infcloud/auth/index.php, the index.php gets downloaded instead of rendered.

Am I missing something, or ist something missing in the docu? Somehow it seems, I must tell the radicale server to execute/render php?