I also have the same problem.
Server: HP Blade BL460c G8 ( E5-2620 with 96GB RAM ) + FlexFabric VC
ESXi Version: VMware-VMvisor-Installer-5.1.0.update01-1065491.x86_64.iso
VCSA Version: VMware-vCenter-Server-Appliance-5.1.0.10100-1123965_OVF10.ova ( using embed. vPostgres database )
The hardware and software are all new installation.
VI Client and VCSA are in the same subnet.
I can use viclient connect to ESXi directly and no any problem.
I also can add ESXi host to VCSA, but just 1 ~ 2 mins ago, the ESXi host change it's status to "not responding"
I can ping ESXi from VCSA, and the related ports ( ex: 443 , 902 etc. ) are ok.
I don't know what can I do, the state is so strange. 