I've found sometimes the same type of error: the game looks for a non-existant texture, which isn't even cited in descr_model_battle.
It happened to me when a model has not assigned a texture for a faction, let's say that the model sarmatian_archer.cas is used for dacia faction (look in export_descr_unit), but there is no texture indicated for dacia in descr_model_battle.
In such case, the game seems to look for a "default" texture, and this information is included in the model. When I open a model in 3ds max, and try to put a bitmap texture, there is always a file name coming by default. You can choose another one, but the model has some kind of information in this regard.
In my experience, look for the unit using that model in the factions you tried. You should find some texture information lacking in descr_model_battle.
I hope this helps.
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