I have also published a bug report for that one, but here is the problem:
I’ve followed this tutorial to create blurry outlines: Rendering Soft outlines in Unreal Engine - Tom Looman
And it worked in 4.19.
But now in 4.20 it doesnt, there are no compiler errors, but there are these warnings in the output log:
You can download the testproject here: CustomMaterialError
To reproduce the error, just open up the showcase map and you’ll see that the post process material, which uses a custom material funtion, is not compiling.