If the problem is happening to everyone, I'd suggest restarting the Unity server. Check the app logs in the event viewer to see if there are any errors or warnings from the time you clicked on Start Unity. Make a test call and if you still see an error, try rebooting the server.
Again, check the app logs for warnings/errors from the time that the server restarted.
HRESULT 0x80004005 translates to E_FAIL which means cannot access, due to permission or does not exists (among other causes).