I landed on this thread via internet search. Apologize for the necro but did want to offer a suggestion/idea that I haven't seen discussed. Not an OMV user yet so I have not tested this, but it did work for me on Debian Stretch after struggling to get NextCloud running. Will probably test in in the next few days on OMV, but in case somebody has time to beat me to it, would be curious! No promises that this won't break something and recognize that I am a novice.
NextCloudPi (https://github.com/nextcloud/nextcloudpi) or (https://ownyourbits.com/2017/0…ready-raspberry-pi-image/) bundles everything together and looks reasonably secure (perhaps more secure for the average user trying to self-configure). You get Apache, MariaDB, some reasonable defaults, and a web interface to manage a whole bunch of "plugins" including LetsEncrypt, Fail2Ban, and what not. Specifically checks for Debian 9 to run the install command. Basically runs all of the apt commands and configuration as a script. You'd need to run it in a VM or a container.
I apt installed LXC on Debian Stretch, started a new Debian Stretch LXC container, and ran the install command (CAUTION: I assume this would completely bork your OMV install if you ran it on the host rather than in the container)
I don't see any reason that wouldn't work on OMV4. You'd apt install lxc libvirt0 libpam-cgroup libpam-cgfs bridge-utils
Start a Debian Stretch container, attach to the container, double check that you are attached to the container, and run the NextCloudPi install command. Come back a few minutes later, set passwords, tweak configuration, and you're off.
Slightly trickier (but still not as bad as standing up NextCloud from scratch, IMO) will be configuring storage/permissions and networking. Out of the gate, the container will be NATed and you'll have to either port forward through OMV or configure a bridge interface to expose the container to a real IP. Those seem pretty solvable though. Highly recommend reading through the Debian LXC page (https://wiki.debian.org/LXC) and the LXC developer documentation (https://linuxcontainers.org/lxc/getting-started/).
4/13 Edit:
Took a bit more tinkering than I expected, but not too bad. Got LXC running on OMV, set up a network bridge, and tested running a privileged container. I'm not smart enough to know whether that is any greater threat than installing NextCloud and its components directly on OMV. Anyway, took steps to run unprivileged LXC containers, created a new Debian Stretch AMD64 container with sudo, and executed the NextCloudPi install command. 10 minutes later and I have a working NextCloud web interface.
Ideally I would like storage to be shared between the OMV user and the NextCloud user, but have not figured out the best way to make that work with permissions and unprivileged LXC container limitations. Will probably update this if I figure that out. If anybody wants specific steps I'll try to retrace.