Is your kRO grfs up to date and is your custom grf inserted in your data.ini?
Or if you use data folder you need to apply the patch "Read Data Folder First".
Updated to 2.4.28, fixes file corruption caused by back-ports from trunk, that were added to 2.4.26. Please read follow up instructions, if you update from 2.4.26 or 2.4.27.