While adding an ESX host to a vCenter I received the following warning:

Configuration Issues
Virtual machine creation may fail because agent is unable to retrieve VM creation options from the host (vmodl.fault.HostCommunication).

First make sure you can successfully ping the specific ESX host FQDN from your vCenter server.

C:\Users\Administrator.lab>ping esx01.lab.local

Pinging esx01.lab.local [192.168.1.100] with 32 bytes of data:
Reply from 192.168.1.100: bytes=32 time<1ms TTL=64
Reply from 192.168.1.100: bytes=32 time<1ms TTL=64

Second log on to your ESX host via SSH and make sure you can ping the vCenter server FQDN.

~ # ping vcenter.lab.local
PING vcenter.lab.local (192.168.1.50): 56 data bytes
64 bytes from 192.168.1.50: icmp_seq=0 ttl=128 time=0.374 ms
64 bytes from 192.168.1.50: icmp_seq=1 ttl=128 time=0.252 ms
64 bytes from 192.168.1.50: icmp_seq=2 ttl=128 time=0.248 ms

If any of the above fails the main problem is a DNS issue.

In case both work successful try restarting the Management Agents on the ESX host.

The ESXi Shell can be disabled by an administrative user. See the
vSphere Security documentation for more information.
~ # services.sh restart
Running vmware-fdm stop
Stopping vmware-fdm:success
...
Please use /etc/init.d/sfcbd-watchdog start
Running vmware-fdm restart
Starting vmware-fdm:success

~ # 

Next try reconnecting the specific ESX host. This should restore the connection and resolve the issue.