Seems like I found the solution - but I am not sure yet. Will look if the reboots are consistent and stable, because my first OMV-installation rebooted sometimes fine and then suddenly it wouldn't work anymore for some reasons.
I was asking for help in a few #debian IRC channels and somebody immediately suggested to try different "reboot="-kernel options.
ZitatAlles anzeigenreboot= [KNL]
Format (x86 or x86_64):
[w[arm] | c[old] | h[ard] | s[oft] | g[pio]] \
[[,]s[mp]#### \
[[,]b[ios] | a[cpi] | k[bd] | t[riple] | e[fi] | p[ci]] \
[[,]f[orce]
Where reboot_mode is one of warm (soft) or cold (hard) or gpio,
reboot_type is one of bios, acpi, kbd, triple, efi, or pci,
reboot_force is either force or not specified,
reboot_cpu is s[mp]#### with #### being the processor
to be used for rebooting.
At first none of them did work (not even reboot=efi). Wen't to the UEFI and enabled Legacy-Mode again (Linux has no problems with switching UEFI/Legacy-mode on the fly it seems) and it booted up fine.
Opened up a terminal per SSH, logged in as root and:
nano /etc/default/grub
changed GRUB_CMDLINE_LINUX="" to GRUB_CMDLINE_LINUX="reboot=bios"
Saved everything, ran "update-grub" and rebooted without any issues so far. Hopefully it will stay like that