Yesterday, 2017-08-08, Nutanix release a minor version of Acropolis Operating System (AOS) formerly known as Nutanix Operating System (NOS) and this means the most current version is 5.1.1.3.
As you might be aware of the recommendation if running Nutanix Self Service Portal (SSP) has been not to upgrade to 5.1.1.1 & 5.1.1.2 from 5.1.x & 5.1.0.x. However, now you can successfully upgrade to latest and greatest version without any of the current problems seen with SSP.
And as usual, if you upgrade to AOS 5.1.x from AOS 4.7.x or 5.0.x and running vSphere make sure to check out the Nutanix Knowledge Base article 4332
AOS 5.1.1.3 can be downloaded here and the release notes can be found here.
Enjoy the new release.