By this time, I’m pretty sure, that the problem lies in the different database structures in FOG 0.32 and 1.3.x. After I imported the hosts from 0.32, not only are they not listed on the web interface, but I also can’t add a host manually anymore, so the import from 0.32 apparently messes up the databases somehow. I installed phpmyadmin to check the databases after the import and the field hmHostID from hostMAC is completely empty. If you add a host manually (it works before the import), this is not the case and it shows up properly on the web interface.
I guess, as long as someone doesn’t write something up, that particularly makes it possible to import from the old into the new database structure, we won’t be able to successfully import the hosts from 0.32.
If someone comes up with a way, that is working, please share.