I had the same problem -- This has to do with the sorting param in the shader. The message is cryptic at best -- I had to open up the source for the built-in default skybox shader and compare with the code generated by shader forge.

However once you spot the difference, this is EASILY fixed --

Under the shader properties in shader forge
- Select: Blending
- uncheck "Auto sort"
- select Background (1000)

The warning will go away.