Solution
The issue was caused by a bitness mismatch. TestStand 64-bit cannot interface with LabVIEW 32-bit when using the LabVIEW Development System adapter.
To resolve this, either revert to TestStand 2014 32-bit, which matches the LabVIEW 2014 32-bit environment, or use TestStand 2014 64-bit together with LabVIEW 2014 64-bit to ensure compatibility.