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This happens because OMSI has an addiction to DDS textures... in that if given the choice between two identically named textures, one DDS and the other not, it'll choose the DDS one even if it's been explicitly told not to. Version 2.1 saw me switch from TGA to DDS for both of these items, so the problem arises where if an older (2.0.0 to 2.0.6) version is installed over the top of a newer version (2.1 onwards) then OMSI uses the wrong texture, giving rise to the problems you see above.
To fix this, just delete the installation and reinstall the just one version of the pack.
To prevent it happening, simply don't copy over the included copy of my pack when installing a map if you've already got an installation.
Click on the bit where it says Road-hog123 said: ^ in the quote, that'll take you the actual post I quoted. It's in the discussion thread for British Road Detailing Pack 2, there's a download button up at the top just like every other release on FF.