Ok I finally solved it with the help of you guys!
The problem was in the attribute locations in the vertex shader. NV automatically generates those according to the order in which the attributes where defined, while ATI needs to have the explicit layout(location = index) in front of each attribute. Also I switched to #version 330 otherwise those layout() identifiers are not available. The version change is no problem since i want to implement a deferred renderer in the next step anyway.
Again, thank you for your help!