I’m sure everyone here has suffered this bug (also known as the YAAP bug) from time to time.
However, if you have a second account under another username (using the same password is useful), you can actually force your way onto the server to tell the staff to kick your first account.
When you do this, you’ll see a message saying “You are already playing on this computer!”
But…it will let you log on anyway.
Do so, and then tell a staff member to kick your first account.
Then, log back onto your first account.
Nice tip im sure it will be very helpful to anyone with the issue 
Alternatively, you can also wait 5 minutes and the issue will resolve itself without needing external assistance. Understandibly inconvinent, especially when you have buddies in a dungeon that need you to return ASAP cause they somehow set off a monster house, but it’s another option to add to your troubleshooting pool for this issue.
But…5 minutes is an eternity in some dungeons, and can spell the difference between success and utter destruction. Sometimes logging onto your second account takes care of YAAP by itself, with no need to ask for staff assistance.
Either way, it (usually) takes less than 5 minutes to use the YAAPbuster method I described in this thread.
The 5-minute wait and posting on forums have been used to get around YAAP for ages, but are far slower than this method I’ve described.
The reason is due to a beneficial bug in the “You are already playing on this computer!” message (PRECISELY this message, NOT “You are already playing!”), which still allows you to log on (and thus ask for your YAAPed account to be kicked).
Whenever I get the ‘YAAP’ bug I just restart my computer.
Works like a charm.
Might not be as effective for people who have really slow computers.