Failed completely with opaque upgrade message

latest/stable: 19.0.0snap1 2020-06-24 (21796)
Ubuntu 18.04.2 LTS
Version: 2.45.1+18.04 (snapd)

The issue you are facing:

After a problem with the hosting container (LXC) crashing due to running out of disc space I have the container back up and running.

When I run the status command I get…

root@liked-pheasant:~# nextcloud.occ status
Nextcloud or one of the apps require upgrade - only a limited number of commands are available
You may use your browser or the occ upgrade command to do the upgrade
  - installed: true
  - version: 19.0.0.12
  - versionstring: 19.0.0
  - edition: 

Running the upgrade command:

root@liked-pheasant:~# nextcloud.occ  upgrade 
Nextcloud or one of the apps require upgrade - only a limited number of commands are available
You may use your browser or the occ upgrade command to do the upgrade
Set log level to debug
Turned on maintenance mode
Exception: Updates between multiple major versions and downgrades are unsupported.
Update failed
Maintenance mode is kept active
Reset log level

How can I resolve this?