It’s not a bug. You have enabled two mods that want to change the same graphic files. If you want to play Autumn Rampart without this bug, you must disable “HotA balance mod” submod under Horn of the Abyss mod. As you know HotA fixes some SOD graphics file and brings changes in balance (in Conflux for example). So, two mods try to change the same graphics files in the same time Unfortunatelly, between these mods there’s conflicts and you must disable one of them.
The same conflicts will happen with many town modifications mods and hota balance submod - Sand Tower, Red/Snow Castle/Another Rampart.