Allow Anonymous Access

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.

This means precisely what it says. Windows will not allow you to connect to the same server using two different usernames at the same time. This is a trick that gets you two user names - if you use the IP address for one logon, you can use the nasname for a second one. That's because Windows doesn't know they are the same machine. That is why \\hostname fails, and \\ipaddress works in your case - you have simply run across that trick.

Check the credentials manager in the the control panel, and either edit the credentials to use the appropriate user name, or just delete them.