Unable to add or open a test project

Question

I used to be able to deal with tests projects. Today if there is a test project in my solution I can't open it. It's represented like a solution folder an dI can't open it.
If I try to add a new test project I get the following message :

Value cannot be null.
Parameter name: path1

I'm using VSTS 2008

 

Thanks for your help, I'm lost.
NOTE : My visual studio is half english/half french...