It appears the issue with 10102 is brought on by a disconnection from the server. The server however believes your character is still logged in, therefore you are presented with this error.

Attempt 1: If you have a character on another server, attempt to login with that character.

IF you are met with 1017 errors.

Attempt 2: (The fix that worked for me) Roll a new character on an open JP server (for me this was Fenrir).

I went through the main cut scenes till I was able to see other players on the server (some have said this is not necessary). I then exited the game, logged back in through the launcher, and logged directly into my character (on Cactaur) without even seeing a 1017 (some may not be this lucky). If you however do see a 1017, this means it no longer thinks your character is logged in, which is a better sign than the 10102.