you probably have some out of date skins that were in place prior to most of the latest PBR shader updates that began a year or so ago. If you have ever used the old SIMTEK GT3 mod, that may contain out of date skins as well. One car may be causing conflicts with others. I've loaded two GTE/LMP2 sessions without any of those issues, so I don't think it's the current DLC that is causing the problem.