This was resolved overnight - I am not sure how, but now the volume shows again normally.
However, it opened up a new problem with Nextcloud, which I have posted in a new thread there.
@Neluser Good idea. But it's quite complicated to automate things like this, that's what it's not done. But something to keep in mind. In the meantime, following https://docs.cloudron.io/storage/#docker-images is the way to go.
