Good news! I've identified the cause of the CTD !
I've recognized a strange bug that caused the game to crash every time a very small number of units tried to enter a community.
Here, the "Tribal Chieftains" have a unit of just 3 soldiers. As soon as they come into contact with an enemy unit, the game crashes.
Two solutions to avoid this problem:
1- Code the game - I don't know if this is possible - to forbid any unit with less than 4 or 5 soldiers.
2- Delete all double entries for officers in the EDU (that's what I did for the unit in question so that the battle didn't crash).
Now battle no longer crashes the gametype Ducal Spearmen
dictionary Ducal_Spearmen ; Tribal Chieftains : tribal unit, factional
category infantry
class spearmen
voice_type Heavy
banner faction main_infantry
soldier EE_Spearmen, 56, 0, 1.1, 0.30
officer lithuanian_noble_infantry_ug1
officer russia_captain_early_flag
officer russia_captain_early_flag delete this line
mount_effect horse +4, camel +4
attributes sea_faring, hide_forest, can_withdraw, is_peasant
move_speed_mod 0.95
formation 1.0, 0, 2.0, 2.0, 5, square, schiltrom