When configuring the SharePoint File Share Library web part, if the account/password being used in the Access Account does not have the correct permissions on the Web Front End servers in the farm, one of the following errors will occur:
"The username and password combination cannot be found."
"Access to the path \\xxx is denied"
Confirm that the account has full control in the Share permissions and the NTFS file system permissions.
If it has already been determined that those permissions are in place, then verify that the user account has rights to log on locally to the server(s) running SharePoint (not just the server hosting the Share).
Go to the Local Security Policy in Administrative Tools, as shown below, or by running secpol.msc from the Run line in the Start Menu.
This will need to be done on each Web Front End server in the SharePoint Farm to ensure reliable connectivity, and if at any point the permissions are revoked or lost you will get the same error when you attempt to open a file. At that point, these steps will need to be repeated.