How to set up WebDAV (with wfsctl) on macOS High Sierra with Server 5.6.1?
And ... I found the solution (so far).

The issue was that I did not use Wiki Server before on macOS Sierra + Server. Wiki Server was turned off. However, to enable WebDAV sharing on macOS High Sierra + Server, you need to activate Wiki Server in Server.app and turn WebDAV sharing on.

The tip was found on Tod Olthoff's blog: http://www.toddolthoff.com/blog_files/c6e4e39502d35b674693ad80ac2ca042-445.html

So, the only thing I had to do to get my wfsctl managed WebDAV shares running again was to turn Wiki Server on and WebDAV sharing on in Wiki Server.