I had this same issue and it came from not replacing the [Fonts] section correctly in the Fallout.ini in the Folder, Documents -> My Games -> FalloutNV After installing the Darnified UI MOD just replace the existing Font section with this: [Fonts]sFontFile_1=Textures\Fonts\DarN_FranKleinBold_14.fntsFontFile_2=Textures\Fonts\DarN_FranKleinBold_16.fntsFontFile_3=Textures\Fonts\Glow_Monofonto_Medium.fntsFontFile_4=Textures\Fonts\DarN_Sui_Generis_Otl_10.fntsFontFile_5=Textures\Fonts\Fixedsys_Comp_uniform_width.fntsFontFile_6=Textures\Fonts\DarN_Sui_Generis_Otl_13.fntsFontFile_7=Textures\Fonts\DarN_Libel_Suit_Otl_24.fntsFontFile_8=Textures\Fonts\Glow_Futura_Caps_Large.fntsFontFile_9=Textures\Fonts\NVFont_Test.fnt Hope that helps you.
