hallo danuel,

had the same problem with ubuntu 12.04. at the moment no solution, but have a look at

thread: problem-uploading-ext3-ubuntu-image

We deployed Ubuntu 11.10 with fog.

Installing GRUB did not work.
We had to boot every PC and install GRUB per hand (grub-install /dev/sda).
This had to be done once. Cloneing the second time worked. We gess, that the MBR is not wirtten correct.

After Upgrading from working 11.10 to 12.04 deploying the same! host was not possible! Partitions did not change. (very strange?)

After trying several partionings:

/ on /dev/sda1
swap on /dev/sda5 logical

could be deployed, but cloned maschine has no swappartition.