Well, we received a lot of complains of people not being able to install eBox/Zentyal from an USB stick. We tested by ourselves and it was true, the installer gave a "CD-ROM not found" error or something like that on the first stage. Finally we discovered a "cdrom-detect/try-usb = yes" boot flag (which is the change in 2.2-rc2) and with that it seems to work.
christian, regarding to your issue, I have no idea, everything seems ok
could you try to repeat the process on a different machine?