In order to get correct red and white curbs for 2005 and 2007 seasons at Spa is to remove these two lines in ext_config.ini:
;RESOURCE_FILE_0 = spa_updated_textures/curbb_updt.dds
;RESOURCE_FILE_0 = spa_updated_textures/curbs_new_updt_2.dds

That's because you probably have the /extension folder from other year variations of the track. Skins are correct but /ext folder "overrides" skins folder in priority. If red/yellow curbs don't bother you, never mind.