vCenter shows alert icons with no explanation

Servers powered off due to power outage beyond our batteries capacity.  It's possible they were up and running before the SAN was too.

I noticed that the guests with alerts were isolated to certain ESXi 5.0 hosts, and it was all of the guests on those hosts.  Some internet searching found several potential fixes on VMware's community site: https://communities.vmware.com/thread/389772.

Tried restarting the management network, without success.  Also tried disabling and enabling HA which also did not resolve.

What did work was this:

1. Connect directly to the ESXi host through vSphere client.
2. Configuration --> Security Profile --> Services Properties.
3. Restart the vpxa services.  This did not cause the guests to go offline and no outage was noted.
4. After some patient waiting the errors cleared in vCenter.