[Solved] Custom shader in scrollview does not work in 2021

In our project we have to change the stencil value.
As the scrollview's viewport uses the stencil to mask the content, we used to replace the UI/Default shader in this component with a custom one.
We recently upgraded to Unity 2021.1.3.1f1 but it doesn't seem to work anymore.
I downloaded the default builtin shaders for this version, but even if I replace the viewport material with one that uses the code for the UI/Default shader, nothing appears in my scroll view.

 

Update: Nevermind. I didn't use the right shader for the content