Yesterday evening I found an working solution to fix this problem.
Many weeks I tried so many Fixes at the www but nothing works.
Every time when I try to download the App I became an, 403 Error. The Apps seems to be installed, but isn't.
Okay here my fix.
1. Download the APK of the file, where you can't download from Playstore. (were ever you find this file)
2. Installing and open Titanium Backup. Scroll down to the file and open this menu
3. deinstall the previous installed App and go the Playstore.
Know you can see the Button has changed
4. Pay and install the APK.
Hope it helps