There is a known issue due to a bad RS485 transceiver IC. The following is the note from the engineer at Red Lion:

The problem only happens when the remote device presents a resistive load. Customers with long runs of RS485 cables probably never see this as well because the long cable becomes a dynamic load and not purely resistive. In this case it is the 120 Ohm resistor inside the sensor used for termination and the short wire length. This should function under this condition, but there is something wrong inside the RS485 Transceiver IC (SP3483E) that is causing this to fail. We also don’t  see a resistive load when connecting the VT-MIPM-131 to other Sixnet(RLC) devices, because those use a Resistive/Capacitive termination circuit

Meanwhile, we will screen for this issue moving forward. Below is what Engineer mentioned for screening this issue in production – There is an easy way to screen for the issue though. Just short out the RS485+ and RS485- on the VT-MIPM-131 and boot it up. If the RXD LED comes on, then you have a bad SP3483E Transceiver IC.”