Line coupler only passes telegrams in one direction.

In a project I have 2 lines connected to each other through a MDT Line coupler. So the setup is:

Line 1.1 – Line Coupler - Line 1.2.

I had to create the second line because the client wanted to add 15 presence detectors in a production hall. All the presence detectors are on line 1.2, alle the relaismodules which switches the light on/off are on line 1.1.  In the beginning this seemed to work fine but suddenly some detectors on line 1.2 appeared to skip detections. Sometimes the lights worked great, sometimes not. And this randomly, not always the same lights.

Performing a groups monitor I noticed that not all the telegrams from line 2 did appeared on line 1 anymore. When doing a bus monitor connected on line 2, I noticed that there was a constant stream op telegrams on the bus, telegrams without source address, destination addres,… It seemed like empty telegrams but constantly like congesting the line and Line coupler. These empty telegrams stream did not appear on line 1.

After a lot of searching, I finally replaced the line coupler with a new one (also MDT) and also replaced the power supply 160mA by  a power supply 320mA (Zennio). I had to replace both to get rid of the empty telegrams. So far so good. But then I noticed that the telegrams from line 2 did pass to line 1 but not the other way around. So the status-telegrams from the relaismodules did not pass through the line coupler to the presence detectors. The routing table of the line coupler seems correct. Even when I configure the line coupler as a repeater (so no routing table involved) and I specified that all telegrams should pass, telegrams do not pass from line 1 to line 2, not one. This did not generate to many problems for the client though for a short while.

But now the client tells me the lights and presence detectors are again failing at moments. Because the telegrams are not passing from line 1 tot line 2 and my ip gateway is connected to line 1, I can not perform a bus monitor from home. And before I go to the clients location, and I can not see a solution, I wanted to see if someone could help me out here.

I have contacted MDT about this and sent them my configuration (ETS) and they can’t find a problem with the configuration.

So my question is:

1. Did anyone ever see such a constant stream of empty telegrams on the bus?
2. Does anyone know why the telegrams are only allowed though the line coupler in one direction, even as repeater?

Thanks