Transparent object with png texture renders grey my object with a transparent material and a png texture with alpha channel renders grey instead of transparent.

My setup looks like this (see pictures below)
I baked a shadow map and saved the images as an 8-bit RGBA png file to use it as a shadow-map behind my object.

This is my material setup,
the final render, where the darker grey plane should not be visible, only the shadow
And I already tried to disable shadow rendering for that object.
The last image shows, how it should look like

What do I miss here?
I found the problem. My PNG was not completely transparent where it should be ‍♂️