go to your phpmyadmin
run this query
INSERT INTO `xoops_online` VALUES (0, '', 1080302700, 0, '127.0.0.1');
if not work
drop this table
xoops_online
then run this query
CREATE TABLE xoops_online (
online_uid mediumint(8) unsigned NOT NULL default '0',
online_uname varchar(25) NOT NULL default '',
online_updated int(10) unsigned NOT NULL default '0',
online_module smallint(5) unsigned NOT NULL default '0',
online_ip varchar(15) NOT NULL default '',
KEY online_module (online_module)
) TYPE=MyISAM;