Same here. I get "Authentication is required. You need to sign into your Google Account." Needless to say, I am signed in. Or alternatively I get an error RFC-PC-01. Restarts of the phone don't help. I'm tempted to uninstall the game completely and reinstall. I'll come back and post if I have any joy finding a workaround...
OK so I apparently had a different account open in Google (Play). I made sure that my account matched in Google Play, Games, etc. and I am still getting the "authentication is required" pop-up... sigh...
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https://www.pntbrother.com/google-play-rpc-s-7-aec-0-error-message-while-retrieving-information-from-server/