Ok, I was wrong before. I found the unit which uses that texture: the german version of Barbarian Heavy Cavalry. The file is indeed missing. Surprising no one had this problem before. Here's a hotfix for you for now: Mediafire.com. Just drop it into xc/models_unit/textures. Your battle should then work. If it still doesn't work, or if you can't access the link, let me know.
Thanks for your report!
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