ESXi 5.1 Update 1 Release Notes
<snip>
Hardware Status tab might stop displaying host health status
On an ESXi 5.1 host, Small-Footprint CIM Broker daemon (sfcbd) might fail frequently and display CIM errors. As a result, Hardware Status tab might stop displaying host health status and syslog.log might have error message similar to the following:
Timeout (or other socket error) sending request to provider.
This issue is resolved in this release.
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Try upgrading the hosts to ESXi 5.1 Update 1 and check?