Hmmm, that's weird. Try This link maybe ?
Ok then, I'll look into it and try to reinstall Unity. What is weird though is that we had the problem on both a Windows PC and a Mac laptop. Maybe it has something to do with some weird setting we didn't think of checking. Anyway, thanks a lot for your help ! we'll work on that, and keep you updated if we find the source to the problem (in case someone else ever gets stuck in this situation).