Thanks for your reply! I did some more searching and trying out stuff and I managed to find the problem. I have World of Variety installed, which apparently puts in more sheriff2 texture files in its own folder (update > x64 > dlcpacks > wov > dlc.rpf > x64 > levels > gta5 > vehicles > wovvehicles.rpf). That caused the game to not know which textures to load I guess, thus causing the buggy look. I saw using Natural Vision Remasterd (NVR) causes the same problem! Just delete the sheriff2.ytd files and its all good!
Now about the game crashing part: in an update somewhere in the past Rockstar changed the vehicles.meta for all the Granger vehicles (sheriff2, fbi2). For both vehicles this must be in the vehicles.meta: <layout>LAYOUT_RANGER</layout>, with the update this changed to <layout>LAYOUT_RANGER_SWAT</layout>. REMOVE the _SWAT part!
So finally; its solved! 🙂