Based on the build number, you're running the vCenter Server Appliance 6.0 Update 3b, and based on VMware Interoperability Matrices, your vCenter do not support ESXi 5.1 hosts, see: VMware Product Interoperability Matrices

Not sure if the Matrix is really 100% correct, but I will recommend you really verify if the required network port 902 UDP is open on both directions and consider upgrade your host to a supported version.