Mechlab never separated mission related files from gaming system related files. Therefore a campaign installation from Mechlab contains in most cases gaminf system related files as well and overwrites the ones you installed before. After this the expansion integrity of files is destroyed. I gues you installed a mechlab campaign that incladed the file corebrain.abl.
This file is important for the expansion but not up for a campaign or mission to change it. It is like an application installed into your windows operating system that installes automitically new operating files (here very old corebrain version).
Check your installfolder for pbrain.abl and corebrain abl and delete those. After that install MC2X on top of the existing installation.
There might be other gaming system files that are effected.
This is not a wolfman-MC2X issue but the undisciplined MECHLAB file handling.