Well, I remembered that it was working fine with the same MySQL server before with 1.4.4 when I used Inspircd 1.2. After I upgraded IRCD to Inspircd 2.0 with Denora 1.4.5, that problem occured.
Actually, the MySQL Server is a VE node of a host box which Denora is running on. So I think that, the connection and data sending/receiving to and from MySQL server should be OK.
Anyways, any suggestion on MySQL configuration then?