to Debian Jessie (8.x), and OMV to v3.x. The main issue was that in the move from Debian Wheezy to Jessie, the init-system was moved from SystemV Init to SystemD - with all the changes this implies. Also - quite some quircks needed to be fixed on the Docker-Containers (Before the upgrade, I made the mistake to Apply ACL's to all directories - including the docker-container directory). The Apple netatalk failed, as did the SMB configuration when upgrading OMV 2.x to 3.x. Also, some configuration file tag-names have changed which required a manual interaction.
Last but not least, the HP Microserver, when using an additional drive requires a SD-Card to boot from (it refuses to boot from drive 5, but SD-Card 6 is allowed to boot: Courtesy of HP's ILO crap).

In the end, it took a good part of the long-weekend and some additional troubleshooting in the evenings after that to get all services back up to normal operations. A little goody was that from now on, borgbackup can be used as backup solution. The big advantage is that it requires way less space to backup data than conventional backup systems (yes, even my Stargate backup-scripts have been retired), and it is amazingly fast...