Did you ever figure this out? I've had this problem for 6 months to a year. I was hoping it was just a bug that Ludeon would end up fixing - no luck. Haven't been able to play the game. I've tried to uninstall, reboot, and re-install both RimWorld and Steam several times. I've deleted files out of steamapps/common/RimWorld, Application Support/RimWorld, and Library/Preferences. It's a OS X error ("validateTextureDimensions:1074: failed assertion `MTLTextureDescriptor has width of zero.'"), but something within RimWorld is making . My only remaining theory is that I need to delete the files in the steam cloud, but it seems the only way to do that is to delete files while the game is running. :-(