The co-domain hint worked, but I'm still wondering about two things:
1. Why didn't that problem occur with the other pages?
2. I had a domain limit of 2 set for this web. Though I had only registered the domainname.tld I got the error "domain limit reached" when I tried to register the co-domain. When I set the limit to 3 it worked. How can that be?
Which permissions and owner did the files had after you uploaded it? Did you modify anithing in the ISPConfig sources regarding the permissions and file owners?
The owner/group settings were correct, I think (web13_admin and web13).
The permissions were rw for admins and r for the group for all files/directories, as far as I remember.