trouble shooting Express and Etc gateway

Our theatre setup is: We use a ETC gateway to mix dmx signals between our ION and the lighting Desk of a visiting group...  The Gateway set up is so that with the visitors desk they have access to the intensity value's of our fixture's but not to the NIP's.  We did it this way so the visiting groups can work like they have conventional light instead of dealing with our multichannel led fixtures.

All this worked fine until yesterday... a group came in with a Etc Express 24/48. It looked that our gateway could not handle the output from the Express. For testing our setup i removed the express and used a other lighting desk , and everything worked well. So no problems in configuration in the Gateway or bad DMX cables. So it seems the problems had to do with the output from the Express. But when i bypassed our Gateway and put the output from the Express straight into our dimmers, that worked fine as well. So now it seems DMX output from the express is fine and the problem is the Gateway... 

Any idea what cause this problem ???

Fred Jonkman