That happens when some kind of “Bad HTML” has been fed to the chat window.
Usually this is because somewhere in some of your Report Messages or other report formats, you have a “<” character that is being spuriously interpreted as an HTML tag.
The best option is to find all of those and get rid of them (or replace them with < which will display as a “<” character without causing the problem). The alternative fix is to go to preferences and disable HTML in chat.
You probably missed a “<” somewhere; that’s been the cause of this problem every time it’s been brought up in the forums so far, anyway. Can you share the module (or even just the build file), so someone else can check?