Hi Khelvan!
As i mentioned, the basic mod was from linedog and i hoped that he checked his work good enough but.......

Your answer gave me a precious clue!
"....does sound like something is being built, or trying to be built, that causes a problem...."
I made again an exhaustive check of the files and found some errors which where a building related source for possible ctd.
Two settlement had the building blacksmith but not the needed market which is a requirement to build it.
Two settlements had stone walls but not the required level to build them. And some forgotten tabs after charakter/generals, ..value, value ...you know.

The blacksmith i think was the most fitting source for a ctd after more then two turns. after checking the strat of the vanilla imperial campaign i discovered that Hyperborae has also by default a blacksmith without the needed market!!!!
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blacksmith requires factions { barbarian, ct_carthage, eastern, parthia, egyptian, greek, roman, } and building_present_min_level market trader

region Hyperboria: building type smith blacksmith !!!!!!!!!
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Seems that also CA made that error with the settlements building setup.
Also i can not realy understand why CA has no line for a texture in the descr_model_battle for a lot of the egypt and others, about six units!?!

Now i guess if there are more then one error with your files, the show err function does not work because it can only react on one error at the time.

My next test of the campaign worked error free for more then four turns :O)
That way i learned that we have to be very cautiuos to prevent a copy of errors from others work into our own.

Khelvan, again thank you very much!
Maybe you like to try my work yourself? let me know your mail adress and i can send you the provincial campaign.