Whenever i transfer a VM from one datastore to another i keep getting "Authenticity of host's SSL certificate is not verified"as the error message. I also got the same error message couple of other times. I wonder that we have to install ssl certificate in the ESXi Host.
Check if the ESX(i) certificate is not expired (In a Browser enter the IP of ESXi and Validate the Certificate).
If you see a Expired certificate then follow below steps.
1. Backup Your existing Certificates (Backup Folder /etc/vmware/ssl)
2. Run below command in ESXi shell.
3. Reconnect from vCenter.
Let me know how it goes, All the best.