I think you need some kind of base bodypart. Right now I think the way you're using connector tags is what's throwing errors.
For example, you have Frame (which seems like it would be a good "base" bodypart).
Connectors aren't "these two things are connected" so much as "stick one of these onto each of the things it connects to".
With Frame and Cabin for instance, Cabin is connected to Frame. That's fine and makes sense - stick a cabin onto the Frame body part.
But the Frame body part has connections to Cabin and others, so the game is trying to stick a Frame onto each of those parts.
I'd try treating the Frame part as a "base" part and take off all of its connections - [BP:FRAME:frame:STP][SKELETON][CONNECTOR]
Now when the game tries to build your creature you start with a frame, and then each bodypart with CON:FRAME gets attached to the frame.
EDIT: Might be a good idea to stick [UPPERBODY] and [LOWERBODY] on FRAME as well, though I am not sure if this is necessary.
EDIT2: Looked over your bodyparts and there are others that are connected to something that connects to them. Pretty sure that's causing problems. PLOW is connected to anything that's a HEAD and also the Engine Frame, Engine Frame is connected to anything that's a HEAD and itself and the engine (eep, connecting to itself definitely isn't good).