Steam Leaderboards 4.26

Hey there,
I tried the last weekend to bring leaderboards into my game on steam, but it isn´t working, I have no idea what I missed…

I did everything which is listed there and used the built-in steam stuff:…eam/index.html
Achievements are working now, but not the leaderboards, it fails always to write an entry…

Anyone did this before and knows what I missed?
Is there any additional setup I need to do, like in defaultengine.ini?

Would be nice if anyone could help me ^^