I know this topic is old, but I wanted to post that I was having this same problem with a Windows 2000 server & Symantec Backup exec. I was using the server name & fixed it by changing the server name to the servers IP address. The error I got below disappeared and I was able to connect:
"Multiple connections to a server or shared resource by the same user, using more than one user name, are not allowed. Disconnect all previous connections to the server or shared resource and try again."
Thanks for the Thread.
Josh Delcore