We have simply clicked the admin upgrade button.
Did not expect this:
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Now, while this is a bit discouraging (what if this happens when the site has a couple thousand users?)… we have no users yet. Lucky. But we can’t get it back up running. There is no discernible error, any step to take to recover what went wrong… unless the ominous automatic notification generated. Who received that notification? What does it say? Why does a simple upgrade of the software send the site into oblivion? How can we fix that?
That is a real risk with every app, including Discourse. That’s why big guys are using a stage server and we smaller fishes are doing backups (site AND database) before every update, but at least before any upgrading.