Since you're able to play on the one character successfully but not the rest, it might be a file corruption in your character settings folders.

Are you playing on PC? If so, move the character setting folders to a different location on your PC and then try to log in on your characters. That will force the game to create new settings folders for them and may solve the problem (though you'll lose your saved settings).

If it works, you can try to copy the old files with your saved settings from the original moved folders into the new folders a few at a time to see if the error occurs again. If it does, you'll know the problem was in the last set of files you moved back so you can delete those and finishing copying the rest back. You might still lose a few character settings but most should end up restored.