While WordPress installs an update, it shows "Briefly unavailable for scheduled maintenance". That's normal for a minute. WP Moo looks again after a few minutes before it counts the page as Offline. When the screen stays, the update stopped halfway and left a file behind.
How to fix it
For your developer
WordPress shows this screen for as long as .maintenance exists and holds a recent timestamp. With WP-CLI:
wp maintenance-mode status
wp maintenance-mode deactivate
The second run of a failed check waits longer for this screen than for other failures, see Page checks.
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