I'm having the exact same problem on my Moto X and my Nexus 7, both completely stock. It started yesterday. I cleared the Google Play cache and data (which didn't help), and eventually removed my Google (gmail) account and put it back. That seemed to temporarily fix it on my phone, but not the tablet, and today it's happening on the phone again too. App updates are still working, and if I tap on an app update in the Notifications it will take me into the My Apps section of the Play Store just fine, but if I try to go up a level to the main Play Store it doesn't work (I get the same connection/Retry error that I get when I open up the Play Store from scratch).