a quick update: we haven't found the root cause of the issue, yet, but we have determined that it only hits hosts running ESXi 6. Previous ESXi version (5.5) or other hypervisors (such as Hyper-V or KVM) are not affected. One option you might want to consider, as a workaround and if you're not using any ESXi 6-only feature, is to go back to ESXi 5.5. I just offer this as a suggestion, I don't know if it's applicable in your environment.
Have you had any other host freezes?