I've tried with
On high-score request success -> display Leaderboard ID
And now the Leaderboard opens (after like 5 seconds on the first try) but it remains blank and return this error: "no internet connection".
I'm connected to internet, I'm connected to Gplay Games and I'm also using a different button to "show leaderboards" (without ID) and this one is working..
I'm using a signed release not installed from Google Play, is that the problem ?
Edit: Got it working, it was because I was using a number instead of a string in the variable for the Leaderboard ID...