This error can happen in rare situations during extreme workloads. It can only happen when new metadata is being created (first-time installation or activation of and previously inactivated module). Restart the add-on and the problem should disappear (on very slow machines this error can happen 3-4 times in a row). In these cases keep restarting until all metadata is created.
In some cases, other add-ons running at the same time can be the cause of this so if restart does not help, then stop all other addons, start our add-on and let it create its data. Once complete you can run the other add-ons as before.
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