Tis another bug, change it back to istunnel 0 attach the track in surveyor then go back to content manager and change back to istunnel 1
Tedious but solves the problem.
Sorry to bump this old thread, but I am having exactly the same issue and have followed exactly the previous mentioned steps but have not been able to solve the problem.
Ok, so the built-in tunnel asset has no errors and warnings, build number 2.9. I clone this asset, and mysteriously the following wild errors have appeared!
Error: Tag 'asset-filename' in the container 'track' is obsolete.
Warning: Required tag 'category-class' was missing and has been set to default.
Warning: Required container 'thumbnails' is missing.
Error: The *.texture.txt file is missing for texture resource 'ballast-ballast.texture'.
Error: The *.texture.txt file is missing for texture resource 'bump-bump.texture'.
Error: The *.texture.txt file is missing for texture resource 'reflect-reflect.texture'.
Error: The *.texture.txt file is missing for texture resource 'track.texture'.
Error: The *.texture.txt file is missing for texture resource 'tunnel.texture'.
Can someone kindly advise please? This is literally the very last piece of a very large puzzle.

Regards,
Nicholas