Thanks! For now, rclone seems to be most promising.
Syncthing doesn't fit my needs, because it has no S3 API, mentioning it for future reference.
And btw, rclone GUI is on the wishlist.
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.
