I have a 3500XL and when I try to connect using the CNA it prompts for the Realm "level 15 access" username/password. When I enter a local user account with the local privileged password it fails stating "Unable to connect. Authentication failed."
I verified that I can access the same device with a telnet session using the same account and enable password.
I am using CNA 5.3
IOS is Version 12.0(5.2)XU