Since the "anniversary loot boxes" a couple weeks ago and continuing after that, even today, I can only log in occasionally-- like once every 6 or 7 times, and it doesn't seem to make much difference as to time of day, or length of time between login attempts, or whether Add-ons are enabled or not.
Once I'm logged in, play goes fine. But if I log out in order to immediately log back in with another character, I either get a 303 error or "Error: An error has occurred. Please wait a few minutes and log in again." Waiting a few minutes makes no difference.
I'm wondering if it's a bug or if the servers can't handle the load?