I added Google Play Games Services support to my board game app and implemented Achievements and a Leaderboard through the corresponding clients in a similar way as the GPGS sample app "Type A Number" did. When debugging with adb and my tester account, these features worked fine. When I released the app into production, the Leaderboard started showing the message "Hmm, something went wrong in Play Games". Additionally, achievements can be unlocked and are being shown properly. But after closing the app, this progress is being lost.
Under Google APIs -> Google Play Games Services I get this feedback of the GPGS API calls (in the production phase):
GPGS method calls
Since the release, I apparently mainly get 401 as a response code according to the metrics.
SOLUTION: Google Play Games doesn't work with app published in PlayStore