I think the best solution is to replace the recruitment of the unit that is causing the ctd with a unit that does not have the mercenary_unit attribute. My suggestion is: steppe missile fistaegfataexsdzhytae
What you have to do is first open up Export_descr_buildings.txt. This file starts with the governor buildings, which are the building that need adapting:
For these buildings (governors_house, governors_villa, governors_palace, proconsuls_palace, imperial_palace ) you will have to change all the lines starting with:
Code:
recruit "steppe missile cavalry sauromatae fat aexsdzhytae" 0
into a line starting with
Code:
recruit "steppe missile fistaegfataexsdzhytae" 0
So just copy-paste the new unit name over the old one.
Oh and make a backup before saving!!