Hi,
first of all, if you feel this belongs in the 'Updates/Upgrades'-section, please let me know or move it.
Now, to my Problem:
i've been running openmediavault:all=3.0.7 on my HP Proliant N54L for a while now, no issues. had omv-extras installed, and today, i wanted to install openmediavault-backup, in order to create a backup of my system drive before i was going to make some changes. turns out, that already broke it....
from logs i can see that the installation triggered the upgrade of several other packages:
python-ldb, python-samba, samba, samba-common-bin, samba-common, samba-dsdb-modules, samba-libs, libldb1, openmediavault
were all upgraded.
After the new webinterface login-page greeted me, i couldn't log in. skipping a bit here:
turns out, openmediavault-engined.service isn't starting anymore: systemctl status openmediavault-engined.service -l
● openmediavault-engined.service - The OpenMediaVault engine daemon that processes the RPC request
Loaded: loaded (/lib/systemd/system/openmediavault-engined.service; enabled)
Active: failed (Result: exit-code) since Wed 2019-02-06 20:58:17 CET; 17s ago
Process: 2223 ExecStart=/usr/sbin/omv-engined (code=exited, status=0/SUCCESS)
Main PID: 2225 (code=exited, status=255)
Feb 06 20:58:17 host systemd[1]: openmediavault-engined.service: Supervising process 2225 which is not our child. We'll most likely not notice when it exits.
Feb 06 20:58:17 host systemd[1]: openmediavault-engined.service: main process exited, code=exited, status=255/n/a
Feb 06 20:58:17 host systemd[1]: Unit openmediavault-engined.service entered failed state.
openmediavault-service also has issues: systemctl status openmediavault
● openmediavault.service - LSB: Basic initialization of the OpenMediaVault system
Loaded: loaded (/etc/init.d/openmediavault)
Active: active (exited) since Wed 2019-02-06 20:24:10 CET; 32min ago
Process: 418 ExecStart=/etc/init.d/openmediavault start (code=exited, status=0/SUCCESS)
Feb 06 20:24:10 host openmediavault[418]: Cleaning up...rm: cannot remove ‘/var/lib/php5/modules’: Is a directory
Feb 06 20:24:10 host openmediavault[418]: rm: cannot remove ‘/var/lib/php5/sessions’: Is a directory
Feb 06 20:24:10 host openmediavault[418]: failed.
? Why is it trying to remove the sessions and modules folders? is it safe to do so? should i 'help' a bit? 
when checking omv-engined -f -d i get at the very bottom of the output:
PHP Fatal error: require_once(): Failed opening required 'openmediavault/object.inc' (include_path='.:/usr/share/php:/usr/share/pear') in /usr/share/openmediavault/engined/rpc/clamav.inc on line 22
object.inc is indeed missing
My first attempts to fix this were revolving around rolling back the upgraded packages, but that didn't work, because the dry-run indicated it wasn' t going to find some of the 'old' package versions anymore.
This led me to discover that apparently i had some sources in my apt list files which weren't working anymore. particularly the ones in the omv-extras-org-erasmus.list made problems, but also some local debian jessie mirror i had used during initial setup wasn't hosting all packages anymore. i've fixed my sources.list* files (i believe, not taking into account the omv-extras), and i've reinstalled latest omv-extras via wget -O - http://omv-extras.org/install | bash
So, i've currently got two thoughts about what went wrong:
hypothesis One:
the way the installation of the backups-plugin was triggered, combined with possible apt-repo unavailabilities, caused the system to force-install incompatible package-versions:
/var/log/apt/history.log:3:Commandline: apt-get --yes --force-yes --fix-missing --allow-unauthenticated --reinstall install openmediavault-backup
hypothesis Two:
openmediavault 3.0.99 somehow doesn't run on my hardware anymore
I'm running out of ideas how to proceed with recovery, so please, anyone, any hints?