Memory is not the same as Storage... Memory stores programs that are currently running and data that's in use, storage is long-term storage, which is with files and folders. Clearing out space on your hard drive won't help fix this problem.
Please make sure you have licht.bmp, light_cone.bmp and light_effect1.bmp present in /OMSI 2/Textures/. If they're all there, then ensure you have the 4 GB patch installed and see if the problem still arises. If they're missing, you'll need to backup your settings and verify the integrity of OMSI.