A whisper to literally any account can crash them immidiately for 30 seconds in game.
This is not a joke.
This is no troll.
I tried it with friends, we could all consistently crash each other 100% of the time.
There are certain conditions (length of whisper , content, etc). But its not hard to find out, and its not anything you cannot find when writing a normal message ingame (the only reason we discovered it was literally ourselves whispering normally to each other).
Possible workaround might be to show yourself offline.
(not being a friend doesn't help and noone- literally noone is safe from that).
Please act ZOS, this is unacceptable , yet again....