I just tried this on my own ork and had the same failure. I then loaded your revised version and also had the failure. Could you confirm the date of the build you are using?Move the motor tubes to the aft most body tube, and all is well. Here is a revised design file. Looks like an interesting design. . . good luck.
Perhaps I need to create a new ork from scratch.
The purpose was mixed clustering. I had originally planned removable blocks so that it could be flown without all tubes filled, and I had also contemplated a mix of B6-6 and C6-5 motors, which have the same average time to ejection, as well as the possibility of flying it with a central airstart. As it happens, it's not practical to fly the rocket on anything but the C6-7, so I could indeed change to a seven motor cluster, and if I have to create a new ork for it, I will. That isn't the case for another cluster I'm now designing, though in that design I am pairing the motor tubes.Hmm, I am not sure I agree. The original file should work IMHO. I would file the bug and let some devs look at it.
A bigger comment I would make is that you should use the built-in cluster mount capabilities rather than specifying a zillion separate motor mounts.