Joining new hosts failed with certificate issues – I was getting certificate issues when trying to join NEW hosts to a new host cluster in this datacenter in vSphere. There is a vCenter setting (vCenter → Configure → Settings → Advanced Settings → vpxd.certmgmt.mode) with a default value of ‘vmca’, and VMware support had changed the value to ‘thumbprint’ which then allowed the new hosts to join the cluster using their default certificates (these were newly installed ESXi 6.7 hosts). Once they were added successfully, this setting was changed back to its default ‘vmca’.