They all dig to the same ip, so I'm assuming that the dns stuff is in order. http://188.8.131.52
gets me to the default debian page, which is good.
I'm gonna go out on a limb here and say that it must be something screwed up in the apache vhosts. I'm not that familiar with ispconfig, but looking in /etc/apache2/sites-available is a good place to start. Check to make sure both sites show up in that directory, then check the configs to see if anything is amiss. Might compare with the one that's working. Also check the directory structure/permissions in /var/www/. I understand that ispconfig makes some symlinks in there, check those too.
Edit: I did an nslookup and I get an answer for derodedraad.net, but not the others. Hmmm lemme think
I see there are default ispconfig pages showing up for both sites, you must be on the right track. :-)