Looks like you still have references to (what appears to be) an older version of the material somewhere in your code that it's trying to load.
This is why it's not recommended to edit material and texture files by hand.
It's also possible that particular material just won't work as a trail. This is not as likely, but anything is possible.
In source, when a material fails to load or is missing, it shows as a purple and black checkerboard. The fact that yours is just *invisible* means that it is loading, you just can't see it.