One of my esxi 5.5 U3 hosts was disconnected from vCenter. The VMs were still running, but reconnecting it to vCenter would result in an vmodl.fault.HostCommunication error.

vmodl-fault-hostcomm.png

I logged into the console and looked at the logs and saw that the ramdisk /tmp was full. I verified that it was full with esxcli system visorfs ramdisk list 1

ls -al /tmp showed that an error log file was filling up the entire ramdisk. I deleted the file and the host automatically reconnected to vCenter.

del-log-file