Hey Friso
What a pain. So, if you disconnect the fibre optic cables from the camera and fire the strobes, can you see the trigger LEDs fire? If so, the trigger is ok. If not, there’s a trigger problem. Try new batteries; make sure the hot shoe is attached properly on both the camera and to the UWT board.
If the trigger fires and you can see the flash, reattach the fibre optic cables and fire the trigger. Can you see a light at the end of the cables? If so, the cables are ok. If not, there’s a problem with your cables. (Bit unlikely both are faulty but you never know)
You say your Retras are ok. Great!
The key thing at this stage is does the UWT trigger flash?