Users/Groups not show(or stuck?) in admin console

The environment is:

OS: Ubuntu 12.04 LTS

Java: 1.6.0_32

Openfire: 3.8.1

Database: 5.5.24-0ubuntu0.12.04.1

I changed the user system to my own user table, following the official guide. It works well in most cases, but offen fails when click "Users/Groups" table on admin console. When I click the "Users/Groups" table, it stuck, seems like waiting for the response from the server, for a long time. But all other tabs and pages are all right now.

I checked the error log, there are lots of below errors but I don't think it relates to this issue.

2013.03.07 09:59:21 org.jivesoftware.openfire.muc.HistoryRequest - Error parsing date from history management

java.text.ParseException: Unparseable date: "2013-03-06T09:18:39+0000"

...

Any body met this problem? It bothered me for a long time.