Nopes, unfortunately this didn't change the behaviour.
I don't even know how to approach debugging shader output, any hints? Also what should be the Item Type of a glsl-shader under Visual Studio 2013? Currently my shader files are set to Custom Build Tool.
check for glGetError is returning zero.
for debugging openGL, you can use CodeXL http://developer.amd.com/tools-and-sdks/opencl-zone/codexl/ which will give you an in-depth look at the resources on the gpu your program has created, and you can try walking through what's wrong. (note, this is an amd product, i'm not sure if it works on NVidia, but it's apparently the successor to glDebugger).