I have the same problem too and since I've reinstalled this machine ten times now I'm starting to get a bit desperate.
When I comment out
RedirectMatch ^/$ /apache2-default/
I only see a listing of apache2-default, sharedip and webalizer. You can see what it looks like at http://friisweb.se
My Vhosts_ispconfig.conf is NOT blank and looks like it is in order. The file is also included in apache2.conf.
I've followed the guide for "The perfect setup - debian sarge", but I get this same problem over and over. All looks good, nothing works.
I will give you all the information you need if you feel like helping me with this.