Google Play Store - "An unexpected error has occurred. Please try again later"
Well this happened to me when I used to install apps from playstore using a slow internet connection. For this I used to get the error notification in the notification bar. I used to long press the notification, goto app info, and 'Clear data' there. This used to work for me as playstore again syncs once opened. U can try if it works but recommend you not to clear data on other 3rd party apps as you may lose you data.