I have pointed the VMware template to my vCenter. There is only 1 ESXi, so not in a cluster. Thus cluster name is an empty string. This showed an error that a group with cluster name couldn't be created for the hypervisor host prototype. So I removed that host group from the host prototype. The the error doesn't show anymore, but discovery still does not work (no new items coming in) although I just executed all 4 discovery rules.