Hey there - in the future whenever there's issues with the game... you may want to check Lodestone first to see if there's been something posted by SE saying they're experiencing issues... especially if there's been +10 posts prior to yours, saying they're having the same issue. SE posted about this yesterday morning informing the community they're being DDoS'd, they just haven't provided an update since: https://eu.finalfantasyxiv.com/lodes...a3ebfcad59d31c

While this doesn't fully explain why you can log onto a main character, but alt characters on the same server (or vice versa), one can assume it's due to the above mentioned issues...