1- export track with 3dsimed (for example) to have it in .fbx format
2- Open with kseditor
3- window materials, below you will see that texture that must be in red, if you do not know what it is, simply click on save .kn5 there it will give you an error with the name of the texture, now you know exactly what it is
4- KsperpixelNM tab click on the three dots, assign any material to that texture.
5- save the .kn5 again, if it gives you the error again, it is the texture that is wrong, the brute force solution is to erase that texture.