A file or folder with the name '' already exists Visual Studio 2012

A file or folder with the name '' already exists Visual Studio 2012

I'm getting the following error whenever I try to add a new file to my
sourcecontrolled VS2012 solution.
A file or folder with the name '' already exists. Please give a unique
name to the item you are adding, or delete the existing item first.
I'm 100% sure that there is no file with the given name.
This started when I tried to install the update 3 for VS2012, which failed
with an error that the update couldn't be installed for some obscure
reason.
After that I've tried repairing Visual Studio which didn't fix this error.
So how do I get VS to add items again?