The last machines are booting again.
As we updated to a more recent Xen version, we hit some kernel bug (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=860236
) which caused us even more delay.
All machines are running now, but still a few of them need to get rebooted.
This will be executed later.
If you have any issues, you can always contact us on firstname.lastname@example.org
Thanks a lot for your understanding!