When updateing to t 7.17 on my Windows Server w/SQL Server the site fails with Error 500 after copying the new files

Hello,

I have a site running on 7.13 installed via the Web Platform Installer on Server 2012 with SQL 2012 and it works fine. When I download the 7.17 latest version and replace everything except the sites folder I get Error 500 on every page. I guess when I am using SQL server I am not supposed to overwrite some other file/folder/module during the upgrade.
Any input what that can be?

thanks

Mike