Landscape material error x3004

this has been driving me crazy all afternoon:

Landscape generated importing a r16 heightmap
Slope mask (to blend between grass and rock) generated with worldMachine and exported to a 2k grayscale png (tried also other formats and sizes, including 2017 pixels)
MatLayerBlend_Standard node works great as long as I have a constant number as alpha
As soon as I try to use the slope texture (with landscape coord UV) it refuses to compile the shader and tells me:
"Error [SM5] Material.usf(1574,64): error X3004: undeclared identifier ‘Local4’

see attached screenshot

any idea on what I could try?


found answer here: