VMware vSphere 6.0 has got some issues, like the several related to CBT and data protection, during the first years of its life. Mostly is finally resolved, but you may still have strange issuesespecially during the upgrade procedure (most when you start from a vSphere 5.1 version).
I notice, after an upgrade, also a strange behavior with vMotion where live VM migration fails with this error:
A general system error occurred: PBM error occurred during PreMigrateCheckCallback: No connection could be made because the target machine actively refused it.
This is not an issue related to vMotion or ESXi, but simly it’s related to a service fault (not visible in vCenter healt status).
VMware KB 2118551 (vMotion of a virtual machine fails with the error: PBM error occurred during PreMigrateCheckCallback) explain that this issue simple occurs when the Profile-Driven Storage service is not running!
To resolve the issue, ensure that the Profile-Driven Storage service is running. Funny that the error message is not clear and funny that the service monitoring does not help.
But probably this was cause by the upgrade process that where not able to start all the service correctly (reboot your vCenter Server after the upgrade remain still a good way to check if all the services start in the right way).