This sounds like my problem...ever since installing 402, I get this error AT LEAST once during each online session. I'll be in the middle of a firefight (it never happens when I'm just walking around or by myself somewhere), and suddenly UT will quit, slowly wiping itself off the screen back to my desktop. Then an error dialog appears, with a ton of game-script type information. The first line always reads something like:
Assertion failed: [file: E:\unreal\220\Engine\UnChan.cpp] [Line:714]
I'm typing this from memory, so it's probably not exactly right, but it's close. That line's followed by lots more of the same stuff. The bitch about this is, it seems to happen 15-20 minutes into a game, after you've worked your *** off to get to the top of the score, only to have it all wiped out by this error! Nothing gets recorded in ngWorldStats for me for that game. I hate it.
If this sounds like your problem (or even if it doesn't), send an email to Epic (I think it's bugs402@epicgames.com) to let them know about it. It will probably only get fixed when they get enough complaints.