Here an (old) 3rd party PHP probably from entropy.ch was installed to /local/usr/bin/. The Sierra httpd.conf was modified to work with the different PHP (and virtual hosts).
While updating to macOS to High Sierra, the Sierra httpd.conf was backed up to httpd.conf.sierra, a new High Sierra httpd.conf was installed and gained control, but won't work with the custom PHP5.
After backing up the High Sierra httpd.conf to httpd.conf.high_sierra:
sudo mv /private/etc/apache2/httpd.conf /private/etc/apache2/httpd.conf.high_sierra
reactivating the httpd.conf.sierra:
sudo cp /private/etc/apache2/httpd.conf.sierra /private/etc/apache2/httpd.conf
uncommenting the httpd.conf line
#LoadModule php5_module libexec/apache2/libphp5.so
and changing the version number from 5 to 7 so that it reads:
LoadModule php7_module libexec/apache2/libphp7.so
renaming the custom /local/usr/bin/php to php5:
sudo mv /local/usr/bin/php /local/usr/bin/php5
replacing the soft link /private/etc/apache2/other/+php-osx.conf (linking to /usr/local/php5/entropy-php.conf) with the High Sierra default /private/etc/apache2/other/php7.conf
and removing any reference to the custom PHP5, apachectl configtest was OK and Apache worked again - now with Apple's PHP7.
Best Answer
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.