XF 2.2 Occasional "Site is currently being upgraded" error messages



my users have recently began receiving a "Site is currently being upgraded" error messages on random page loads. It happens at most once per user per session, and is always fixed with a reload. But nevertheless it's quite annoying. I haven't been able to reproduce the error myself so it's been quite difficult to debug.

But some facts:
  • XenForo is at its latest version
  • PHP 8.0
  • The version in the database and in the codebase match
  • No add-on is in the "processing" state
  • Visiting the /install path doesn't show anything odd, no upgrade is in process
  • I have a few other installs on the same server, and there hasn't been reports on the same problem.
Any idea how this could be fixed?
The intermittent nature of it and the fact there isn't an actual upgrade in progress, sounds like a caching issue.
I have an Nginx fastcgi cache in use, but I've purged it a few times now and the problem keeps coming back. So apparently there is some mechanism that produces the error message without a real need.
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