ERROR POL-5311 Steam Version
I too am experiencing this error after trying to return to Vana'diel. The difference is that I know I have the correct POL and SE IDs and Passwords as I just created the SE account and the POL info must have been correct because the accounts linked and all my characters are there and correct.
I've also tried the silly recommendation of reinstalling the game from scratch and still the same issue. I have a ticket in for e-mail support. If they get back to me with an answer that works, I'll be sure to post it here asap.
/salute
Mil'Bereth