I have this problem too.
I cannot reproduce it. Safedocks works fine after having being inserted here.
I select the safedoc mode, and the safedoc and the marsheler appear on the stand.
This shouldn't happen. You can have either a Marshaller or a Safedock, not both. I tested it and, if I switch from a Marshaller to a docking system, the marshaller doesn't appear, and the docking system works.
But as I've said in my last message, you should indicate the PRECISE, EXACT model you used. Not just "Safedock": there are 9 different models of them. I only tried with a couple of different ones, and they both work.
Also, try to "Restart Couatl" after you made changes to the airport customization.