You may have captured these as part of some bullet points already - 16/ Network connectivity. Make sure that the resource is accessible. It could be api server,

You may have captured these as part of some bullet points already - 16/ Network connectivity. Make sure that the resource is accessible. It could be api server, a controller, or an application pod. 18/ What changed in the sw versions - install/remove/update 19/ Is your DNS working correctly? 20/ Does the pod have right permissions (RBAC)