My first guess is that perhaps the Security updates have broken the DB (as per this blog post) and you haven't re-installed the DB (as per instructions there).

If that's the case then it should be a pretty easy fix. Otherwise, I'm not at all sure TBH... Although FWIW I hope to have all the rebuild of all the affected servers completed and the updates uploaded within the next few days.

I'm not sure if you worked out the issue you were having or not? If not, please try the updated Nextcloud image (v15.1). It was released as part of the updated release - all affected appliances have been rebuilt and re-released as new versions.