Wanting to know what this means

[Faetyl]

A corrupt window placements file comes to mind first.
Try backing up your window placements file the character in question first "SERVER_CHARACTER_eq2_uisettings.ini" then delete it and see if you can log in to the game.

If that doesn't do it then try deleting your eq2.ini and launch the game without running the ThorUI AU.
If that gets you in then the issue is most likely contained somewhere in the thorui folder.

Try those two options starting with the first and let me know what the results are.