The certificate for HiveManager NG has expired on October 23, 2019. As a result, Dell N Series switches running an older firmware or an older version of the HiveAgent, can no longer connect or be connected to HiveManager NG after this date.
The solution is to either update the switch firmware to at least 6.6.0.13, or manually update the HiveAgent to 1.1.29.
More information: https://www.dell.com/support/article/en-us/sln319098/dell-emc-networking-hivemanager-site-cloud-aerohive-com-certificate-expiration-how-to-manually-upgrade-hiveagent-on-dell-n-series-switches?lang=en&ref=topsolutions
