I have observed similar behavior on a perfectly configured new 5.x build. The fix for me was to delete and re-create the vMotion portgroup on the VSS (there are related VMware KBs too). This can also affect DVS (including 1000v) in which case the (undocumented?) fixI have used (at least on 4.x) is typically to toggle the checkbox on the vMotion interface.
Perhaps a reboot, restart of the mgmt agents, or deleting and re-creating the desired PG would have resolved the issue if you preferred or needed to stay on the VSS for example.