I have the same problem.
I setup a new access point, changed my SSID. Went to the admin page to set the new SSID in the 6515 and didn't remember the password. Looked on the net for it and found it's supposed to be either 1111 or the SN of the unit. I tried both and neither got me in. I did the full factory restore which I thought was supposed to reset the password to the 1111 or SN. None of them work. I did a factory restore about 6 times over the weekend and the password doesn't work.
I remember when I first got the printer the password was 1111. I changed it to Admin a long time ago.
Why doens't the factory restore reset the password? Nothing I seem to do, even Home button 10secs and 6789, device, factory restore, doesn't clear/reset the password.
Can I get help on this? The printer is about 2 years old and I do not have any maintenance. I don't see any reason to have a maintenance plan on a $400-$500 printer.
When trying the serial number as the password, did you make sure you entered it case senstive? This is very important. Also, any letter "o" are actually the number "0" in Xerox serial numbers.
You will need to call into the support centre 1-800-821-2797 to reset the password, they can generate a code that is needed.