I am having the same problem on Windows 7. What I have found is that if the Windows Explorer is open into the briefcase when I am trying to access it though an application, then I get the message that the Briefcase is in use by another computer. My solution
is to close Windows Explorer and then the application can access the contents of the Briefcase. I also have the same problem visa-versa. If an application is in the Briefcase first, then Windows Explorer cannot open it.