It's made the news yesterday, but in case you missed it, there seems to be a bug within vSphere 5.5U1 that affects all NFS users.
NetApp & VMware (at least, maybe Nutanix too?) are working on the subject.
See the post below for more information:
That may also explain the delay from Nutanix in the support of the 5.5U1?
Did you guys catch this while working no the 5.5U1 support?
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Frequent NFS APDs after upgrading ESXi to 5.5 U1 (2076392)
SymptomsWhen running ESXi 5.5 Update 1, the ESXi host frequently loses connectivity to NFS storage and APDs to NFS volumes are observed. You experience these symptoms:
Too bad, we were to test the 5.5U1 just next week to rollout massively starting next month, guess we'll have to wait...