Everything compiles, but Resharper marks everything in red?

I'm using VS 2008 with Resharper 5.1

Al of a sudden, Resharper shows all classes etc I use in red. As if I didn't refecrence the correct project/dll.

BUT: everything is referenced, and the code compiles.

What is going on and how can I solve this?

I already tried this: deleted the _ReSharper-folder and deleted the 5.1.ReSharper.user-file