ESXi - Agent can't send heartbeats: No buffer space available

Quick post. The other day I saw an alert on an ESXi 6.0 host as follows: Agent can’t send heartbeats: No buffer space available The host was responsive and connected to vCenter Server, and the VMs were running OK on the host. I checked the vmkernel / hostd / vpxa logs, and the only mention of this was a one liner in the vmkernel log which repeated the error message above.