Your LocalFriendCodeSeed_B doesn't match up with the serial number on your console, possibly modified in the past to evade a Nintendo Network ban. To access online services, restore the original using Godmode9.

To do this, download the latest version of Restore-LFCS_B from GitHub and place the script in /gm9/scripts on your 3DS SD card.

When booting up your 3DS, hold START to enter Godmode9. Once it's loaded, press `HOME` > Scripts > Run script > Restore original LFCS and try the first option. Then, reboot your console and check the Friends List application.

If the error code has disappeared, you've successfully fixed this error code. If it still displays, go back into Godmode9 and keep testing each option until you find something that works.