Today I see this error message on one ESXi5.0 host:

The number of heartbeat datastores for host is 0, which is less than required: 2

No any VM is running on the host by DRS or HA, VMware KB gives a solution but too complicate.

Re-configure HA can fixes the problem.

Right click the host -> Click Reconfigure for vSphere HA -> Waiting HA configuration complete.