Okay, I haven't been able to fix it. Time to start from scratch and reinstall. Fortunately my Nextcloud only has about 6GB of data, so it's not a complete disaster.
I would like some advice about best practices when working with OMV, because what I did this time was obviously wrong. What's the best way to install OMV (I can do it through the Softy tool on Armbian) and what should I *avoid* doing, so as not to break my setup?