I do not know if the routine you are using for "password from serial number" is working for that fw version on that device.
Your best bet is to use KindleTool (which can generate the diags password from the serial number).
NOTE: Your PW3 is a "dual boot" system.
I do not think the "diags" system should be showing 192.168.15.244, that is almost always the "main" system's address.
And of course, the "main" system has logins disabled for all users of "main".
Which looks just like what you are seeing.