The omv version doesn't matter. Your grub install is not good. There a many reasons why that could happen. Try booting a debian disk and repairing grub.
You were right, the grub install was not ok and I had to use the debian rescue cd. Now it's booting again!. Thanks very much
What I don't understand is why there is no possibility to have a full working backup using fsarchiver. What am I missing? This whole step seems too convoluted.