This worked for me on OMV6 for now, installing the headers. Then everything else worked. For some reason my nginx proxy was not running if nvidia was not running (no idea when or how that became dependent on nvidia working but whatever). So I needed to do something quickly.
apt install linux-headers-amd64
Still intend to do OMV7 update over the weekend, does anyone know if you should uninstall nvidia and/or turn off backports before you do it for a smoother ride?