tschwarz,
Your file is missing the _U image.
No matter, for testing purposes download the attached skycube. It's a Black Skycube that someone requested. All of the directory and file name structures are correct.
If people plan on making custom skycubes to share with the community, I would suggest the get the freeware Click Team Install Creator (what I used on the attached download). It's very simple to use and you can have the installer drop the custom skycubes right where they need to go within the FPSC directory for instant usage.
"Trust no one" --Deepthroat (X-Files)