OK. That was the culprit, thanks for the analysis. The case of external DHCP for the head-node slipped through our tests with QluMan :(
That's the fun when doing software, there are so many parameters you cannot possibly test them all. And that's why static correctness proofs are a hobbyhorse of mine (although they come at a hefty price in terms of development time).
b) Reinstall right away and use static IP for the head.
Reinstalling right now. I'll keep you posted.
Thanks a lot for you help!
A.