User rights management - no group members shown / group count zero using Oracle

With Jetty zip-version of XWiki XE 1.7 Milestone 1:
1) Using HSQLDB the rights management UI output is o.k. (group member list and count)
2) Using Oracle XE (10g) in rights management UI the member count is zero and the member list is always empty.

To enable Oracle XE in Jetty package i copied ojdbc14.jar file to xwiki/WEB-INF/lib and changed the entries in hibernate.cfg.xml to use Oracle database.
After import of xwiki-enterprise-wiki-1.7-milestone-1.xar and login as user Admin new users can be created and added to groups.
Problem: Member count view "0" (zero) and member list is always empty. You can't view or delete members via user rights management UI (edit objects of document works).

Same behaviour with JBoss 4.2.1. And same with Oracle 9.2

May this be a hibernate mapping problem?

No Java-Script error was seen.

On firebug console following AJAX response is received (Oracle XE database):

{
"totalrows": 2,
"returnedrows": 2 ,
"offset": 1,
"reqNo": 1,
"rows": [

{"username" : "XWikiAdminGroup", "fullname" : "XWiki.XWikiAdminGroup", "wikiname" : "local", "members" : "0", "userurl" : "/xwiki/bin/view/XWiki/XWikiAdminGroup", "usersaveurl" : "/xwiki/bin/save/XWiki/XWikiAdminGroup", "userinlineurl" : "/xwiki/bin/inline/XWiki/XWikiAdminGroup?xpage=editgroup", "docurl" : "/xwiki/bin/admin/XWiki/XWikiPreferences?section=Groups" }

,

{"username" : "XWikiAllGroup", "fullname" : "XWiki.XWikiAllGroup", "wikiname" : "local", "members" : "0", "userurl" : "/xwiki/bin/view/XWiki/XWikiAllGroup", "usersaveurl" : "/xwiki/bin/save/XWiki/XWikiAllGroup", "userinlineurl" : "/xwiki/bin/inline/XWiki/XWikiAllGroup?xpage=editgroup", "docurl" : "/xwiki/bin/admin/XWiki/XWikiPreferences?section=Groups" }

]}