I have Creo 2.0.
It might as well been something I did or did not.
I resolved the "glitch" so that I opened all the parts, the skeleton and the assembly one by one and then changed the unit system in each of them separately to be the same (millimeters) and then saved them respectively.
Thanks Antonius for the reply. I definitely will be more careful with the units and the measurement units further on. The change was so abrupt, so I would think it was a glitch, or then I should have done something in a very precise order (and there is no mntion of such a requirement, as far as I know ) or maybe some of the parts was done with wrong units, and that may have caused the erroneus action.