Android games leaderboard broken while testing

I have successfully integrated the Google Games API in my app, and now I can see my leaderboard and achievements as described here and here respectively.

However, when trying to submit a score using this code:

Games.Leaderboards.submitScore(mGoogleApiClient, getString(R.string.play_console_leaderboard_id), 100);

for some reason the leaderboard stops appearing. Actually it starts loading, and then directly disappears. This post seems to describe the same issue, however there is no solution posted.

I know that it has to do with the submitScore because when I create a new leaderboard, I can successfully open it and see the message that no one has reported any score. Then again, if I try to submit a score, the leaderboard again breaks.

Any hints / suggestions on that? Thanks.