Against all odds I found some other people who were also using FullXML (imho the most crappy cms out there, I can’t even open their website properly) .
Well, when I needed to migrate some of these sites to my new Windows Server 2008 version, I got this strange error. Of course the error had nothing to do with the real reason behind it. Thanks to this blogpost I could easily figure out what was wrong:
As the blogpost says, it were the permissions on the databasefolder that are needed to be set properly. If the cms is not able to write to the specific folders, the error will pop up.