Hey Guys ,i was Having same problem ERROR RPC S-7 , AEC-0 with my Galaxy S5, i tried many things then i contacted google and it was really easy to solve this issue with google help. you guys just try it before trying anything else. Steps are given as follows:
Go to settings > Date and time settings > uncheck these options like Automatic date & time, Automatic time zone, then set date and time wrongly and press home key, then go to these settings again and set date and time righlty and check these options Automatic date & time, Automatic time zone again and then try Google play store , i Hope it will work now. Thanks