Hello Marsassassin,
So the reason you are having issues is because of the ‘1-x’ node. I have set up my own material using a combined approach from one of the example materials in the engine and a ‘FuzzyShading’ Node. My approach is just a bit more complex, but gives you greater control over the subsurface strength.
Custom Subsurface Ivy Shader
The thing to pay attention to here is the ‘Alpha’ channel of the diffuse texture is being fed into the ‘Opacity Mask’ input, and the ‘Opacity’ input is being derived from the red channel of the masked texture sample.
Original/Simpler Subsurface Shader
This set up is a bit more straight forward and uses a multiplier and a desaturation node. This allows for a greater range of control for the overall color.
Comparison in the World
The one of the left is the shader I created, and the one on the right is the original/second shader. Both approaches work great, but with the one I created you can convert the constants to parameters if you wanted to create a material instance for customization.
Let me know if you managed to get your SSS to function as intended, or if you are still having some trouble.
Cheers,