Thanks Rafael, I found the problem actually.
It's because we set the jforum link to "", in the SystemGlobal file.
when people trying to post a message, the watchlist notify function is trying to get the jforumLink for sending the email,
because jforum Link is "", so exception is thrown.
the reason we set the jforum URL to "", becasue the smilies
pattern looks like
CONTEXT is jforum URL, and jforum has been deployed with out root application, we dont need the CONTEXT anymore, so set it to "".
[originally posted on jforum.net by shanyangqu]