There is a problem with the HP AMS module on ESXi 5.x, if your installation is affected by this problem, the only solultion might be to shutdown all running Virtual Machines, and reboot the host.
If you want to check if your installation i affect you can se the version of the AMS module here, look under "Software Components":
The affected version are:
If you are running these version upgrade them to 10.0.1.x.
For more information se VMware KB: http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2085618