Occasionnaly, I received the "Unable to load reference library" in the interface. This occurs even if I was not working in the environment. I could have a project loaded for days, work on various other things, and suddenly, when switching to that task, I see that message in the Error list window.

In some cases, I can do a Rebuild Web Site to fix this. But, on some other cases, I have to quit and reenter.

The reference is a DLL. That DLL may be updated in another project window however, another Windows task. But, usually, this does not affect those that are opened. Because, the DLL only gets updated in the specific projects as soon as I will compile then again. So, even if the DLL is updated in its own directory, the copies of its reference in all other projects remain untouched.