I've formatted it again and downloaded the 14.2 firmware (again), suddenly it díd work.
(Maybe the 15.3 or something doesn't?).
The weird thing now is that it shows 'Pocket101' (internal memory) as empty, in stead of showing the system, etc ... (It does open books the normal way however).. :/
So I can't save notes (or add dictionaries, etc).
I'll re-install later.. but not now