I know what you mentioned in your first post. There is a difference between \\computername\username and just username. ;) I myself ran into a similar scenario when I entered just the username and it didn't work. However, entering \\computername\username did work. The key difference in my case was my system was looking to log into the local system with the other system's credentials which wouldn't work. Adding the \\computername\ to the username to log into the remote computer using those credentials. In my case, it was a mapped network drive and that may be the difference.
Clearly, your case had a different cause. Thanks for updating the community. :)