I do think OMV is a great project.
I do think Nextcloud is also a great project.
Unfortunately, adding two great project together sometimes doesn't work, or failed, or end ups being not so successful.
I spend sometimes (2 weeks times, around 20 hours ) trying to make them work together and failed because i am consider myself as a medium linux user and get bored quicly.
So i decided to do my homework and start from a clean debian-9.3.0-amd64-netinst with only the common-tools and openssh installed ( 128 paquets )
It took me around 10 hours to learn/apply/built my system from sratch.
Now, i am able to do whatever i want with it.
I can send you my "notes" with a list of steps i did ( not completes, just giving directions ) ... on request.
final system is:
Nextcloud server 12.0.5 --> 13.0.0 no problems so far
based A8-6500 8GO RAID6 8*6TO
4.9.0-5-amd64 #1 SMP Debian 4.9.65-3+deb9u2 (2018-01-04)
/ apache 2.4.25 --> 2.4.29 / php 7.0.27 / 10.1.26-MariaDB / mysqlnd 5.0.12
/ phpadmin 4.6.6deb4 / Redis server v=3.2.6 / HTTP2 /