This is still something I'm quite not certain about, but I'd like to believe it has something to do with the friendlist.
I've gotten a few reports throughout the time here about players getting a bigger friendlist, and then one day decide to clean out it out, it has happened that once they do this, it gets to a point where the user constantly will crash on that account. I cannot verify if it's character based, or account based, but from what it seems, there's something that messes it up on the whole account.
There just hasn't been enough to find where the issue might lay with this, if it's an ingame glitch or server one, but it could really be either.
Something that people must keep in mind with this odd happening is that there's 2 kinds of it and I know this for a fact:
1st type is that the game just wont work on a certain account, no matter what you try it will crash your game and you're left with an ineffective account, source of the issue I'm not certain about, if it's friendlist, groups, specific character.
2nd type is temporary and can be caused by certain people and their friendlists, once they connect to a map with that user in it, they can cause the whole map to crash for everyone, but this one doesn't happen too often.
The one mentioned above shouldn't be confused, with just being affected by this crash, because if any user try to connect to the map as it's down due to the crash given on type n2, or if they are inside of the map before and as it crashes, they'd most likely be able to join the game with ease again, except that one particular user that caused the crash together with the other one walking into the map.
There's also a third crash but it's completely unrelated to the subject at hand, but I thought I'd explain it.
It's when users cause the crash for themselves, either the computer can't keep up, or if they input a non english key for example. Their game client crashes but everyone else is still connected and play normally, this crash should not be confused with the ones above.