Don't need to post the log, as it was empty anyway and figured the cause of this.
The [Dismounted] Men at Arms/Professional Soldiers (any faction) causes a battle crash after the 1st & 2nd armor-upgrade, cause of a missing head-texture.
The last armor upgrade works fine.
edit: was bored and figured the error with the full log enabled.
for "dismounted_italian_maa_ug1" and "italian_maa_ug1" the following line(s) are invalid and the texture missing for any of these 6 listed factions. (battle_models.modeldb)
If changed to65 unit_models/AttachmentSets/Final Knights Heads_slave_norm.texture 0
then it's working again and the model appears properly with the new armor You could as well copy & rename "...\data\unit_models\attachmentsets\final knights heads_base_norm.texture" to "final knights heads_slave_norm.texture". Am using an updated modeldb with the hospitaller fix, but it is based on 6.4, so the error should be in the base file as well.64 unit_models/AttachmentSets/Final Knights Heads_base_norm.texture 0
Took the liberty to attach the fixed version as well, should be working again with HRE/Sicily/Papal/Venice/Milan/Slave and also includes the Knights Hospitaller Full Plate armor fix.
Extract it, and put the "mods" folder into your SEGA/Medieval II Total War/ folder, or wherever you installed your game.