"Add units-Cultural difference" crash. Why?

Hello all, and it's nice to be in this great site for Total War. I have a question and any advice would be greatly appreciated. In case this is already covered, I am sorry. Although I searched, I failed to find it.

Following the guides published in this site, I am trying to add a new faction. I have done everything correcty, checked and re-checked. The game crashes while loading the campaign map with the message "Attempted to add units in an area controlled by a different culture" or something like that. I have checked the .txt file of startpos again and again, making sure that the faction I'm adding has the same religion and culture with the provinces I have assigned to it. What have I missed?

I noticed that 1-2 factions (like the Byzantines in Naples) have a line that says that it "Forces the unit to be placed". I tried it, and then I wasn't even able to go to the startup, select scene.

Thanks in advance

This message refers to a culture conflict between a province and a unit placed in that province. Unit cultures are specified in the units text.
You most likely tried to place units in a province owned by a different faction than what you believed, or you didn't delete all the old units from a province you gave to a new faction, including spies and such, which are at the end of the file.

Hah! That was it. I just removed from the agents' list an emissary I had placed in my province, and voila! It worked. I still don't understand why the emissary wasn't assigned to my faction, but anyway. What's important is that it now works.

Thanks a lot for the help