This can happen due to too many connections during the creation of metadata.
This error can happen in rare situations during extreme workloads. It can only happen when new metadata are 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 some cases, other add-ons running at the same time can be the cause of this. If a restart does not help, then stop all other addons, start Add-on and let it create its metadata. Once complete you can run the other add-ons as before.