I've now done a clean reinstallation of the ispconfig system and I still have the same problem. I have no errors in /var/log/apache2/error.log, apache2.conf has the ispconfig addons and it includes the Vhosts_ispconfig.conf, and Vhosts_ispconfig.conf looks alright to me.
httpd -V gives the same as I reported earlier.
Seems that no redirection works at all. When I browse the ip-adress I get that same docroot, but I should get the sharedip page, right?