I've had this problem on a few occassions and have narrowed it down to a particular zone (at least for my accounts) Buburimu Peninsula. I can only have one character in the zone at a time, which really sucks. I had to log on to each account and move them from Buburimu before I could successfully log both back into game. It's definately an issue on SE's end and not your ISP's, Routers, corrupted files etc... I hope this helps or sheds some light on the issue.

Take care