So I
FINALLY managed to solve this on my Google LG Nexus 4.
The steps in
Post #181 were only partially successful. Most of the apps successfully updated, but even after that, Google+ and Google Play Music still returned the "RPC:S-5:AEC-0" error.
I have this 1 other app that I deliberately avoided updating for many months because I did not agree with the latest permissions. I outright uninstalled this 1 other app, power cycled, and immediately thereafter all other updates were working again.
This makes me think - did avoiding certain updates cause some kind of "de-synchronization" in Google Play or cause something to get out of whack? Maybe Google Play itself has some compatibility issues with the latest software updates versus older software updates, and it assumes that everything is going to be fresh.
IN SUMMARY, if you are still having this problem, try Post #181's fix first. If the problem persists, try uninstalling any app that has been eligible for updates for a long time, but wasn't actually updated for whatever reason. Reboot and try again. It worked for me.