Gday Marko
I'm using 4 of the Dmx2-18 boards but on a F16v2r, looking at your avatar of your setup it looks like you have the Dmx2-18 plugged into Dmx serial output #1 of the F16v3. I think the Dmx #2 serial output is the preferred port. I had the same issue as you but swapped to the Dmx #2 and all is good.
I'm using 4 of the Dmx2-18 boards but on a F16v2r, looking at your avatar of your setup it looks like you have the Dmx2-18 plugged into Dmx serial output #1 of the F16v3. I think the Dmx #2 serial output is the preferred port. I had the same issue as you but swapped to the Dmx #2 and all is good.