Can not install/download APPS via Play Store to my Tablet ....how can i fix this?
I had to re-authenticate on my tablet, too. I assume Google is trying to detect fake accounts used to push fake-apps into the top apps list.

May be in your case there is a conflict between the cached data and the actual store data. 

You can try to clear the app cache of "Google Play-Store services" app and the Google Play-Store app. 

Additionally you should check your Google account in "Settings" -> "Accounts". Manually perform a synchronization.