So...I am also getting the constant 90002 error and am located in the EU. After all my current experimentation, this is what I found and hopefully if someone can check it out as well to see if we can narrow down something.

1. If log onto a different system, I am able to play my one character that keep getting dc. However, that would mean I can only play an alt char on my actual system.
2. Will disconnect if I get on a mount, try to fly, or even entering a dungeon instance.
3. I am able to play as long as I run on my own legs (means cannot use my own mount unless it is only for 2 seconds) throughout an entire map. However, will not dc if I am using a mount provided by an NPC for a quest or from the NPC porter in town.

Overall, spend gils to move from point A to point B or run on your own legs. So can someone check this out if it's also happening on their account? Thank you.