Back in April VMware identified an issue when running ESXi 5.5 Update 1, the ESXi host intermittently loses connectivity to NFS storage and an All Paths Down (APD) condition to NFS volumes is observed.

You can read more details about the problem at http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2076392

VMware announced today (11 June 2014) that the issue has been resolved.  Download and details are at : http://kb.vmware.com/kb/2076392