it's me again. I have already figured out the problem. Seems my edition of the game is faulty. As I checked the mission file with X-ED the tug group was set to board and deliver cargo to another group but the group was not defined. So once I added that information, updated my CD with the new file, it suddenly works!
Tugs do what they are supposed to do, and I can finally finish the game.
I'm just posting this info in case anybody ever runs into the same or similar problem as me. This editor really works like a charm, can recommend it.
Why my edition really is faulty is a mystery to me.
Thanks to anybody who looked into this thread.