解决办法一
直接删除内置的计划任务，如果删除不了的话，打开数据库删除pre_common_cron表这个计划任务，或者将后台计划任务中的可用性取消勾选。
解决办法二
以上办法都无法解决可以考虑重置这个表pre_common_cron，到数据库执行如下字段即可。
DROP TABLE IF EXISTS pre_common_cron;
CREATE TABLE `pre_common_cron` (
`cronid` smallint(6) unsigned NOT NULL auto_increment,
`available` tinyint(1) NOT NULL default '0',
`type` enum('user','system','plugin') NOT NULL default 'user',
`name` char(50) NOT NULL default '',
`filename` char(50) NOT NULL default '',
`lastrun` int(10) unsigned NOT NULL default '0',
`nextrun` int(10) unsigned NOT NULL default '0',
`weekday` tinyint(1) NOT NULL default '0',
`day` tinyint(2) NOT NULL default '0',
`hour` tinyint(2) NOT NULL default '0',
`minute` char(36) NOT NULL default '',
PRIMARY KEY (`cronid`),
KEY `nextrun` (`available`,`nextrun`)
) ENGINE=MyISAM DEFAULT CHARSET=gbk AUTO_INCREMENT=22 ;
INSERT INTO `pre_common_cron` (`cronid`, `available`, `type`, `name`, `filename`, `lastrun`, `nextrun`, `weekday`, `day`, `hour`, `minute`) VALUES?
(1, 1, 'system', '清空今日发帖数', 'cron_todaypost_daily.php', 1416342963, 1416412800, -1, -1, 0, '0'),
(2, 1, 'system', '清空本月在线时间', 'cron_onlinetime_monthly.php', 1414791807, 1417363200, -1, 1, 0, '0'),
(3, 1, 'system', '每日数据清理', 'cron_cleanup_daily.php', 1416357390, 1416432600, -1, -1, 5, '30'),
(5, 1, 'system', '每日公告清理', 'cron_announcement_daily.php', 1416346616, 1416412800, -1, -1, 0, '0'),
(6, 1, 'system', '限时操作清理', 'cron_threadexpiry_hourly.php', 1416357391, 1416358800, -1, -1, -1, '0'),
(7, 1, 'system', '论坛推广清理', 'cron_promotion_hourly.php', 1416347376, 1416412800, -1, -1, 0, '00'),
(8, 1, 'system', '每月主题清理', 'cron_cleanup_monthly.php', 1414860761, 1417384800, -1, 1, 6, '00'),
(9, 1, 'system', '道具自动补货', 'cron_magic_daily.php', 1416347946, 1416412800, -1, -1, 0, '0'),
(10, 1, 'system', '每日验证问答更新', 'cron_secqaa_daily.php', 1416357391, 1416434400, -1, -1, 6, '0'),
(11, 1, 'system', '每日标签更新', 'cron_tag_daily.php', 1416347946, 1416412800, -1, -1, 0, '0'),
(12, 1, 'system', '每日勋章更新', 'cron_medal_daily.php', 1416347947, 1416412800, -1, -1, 0, '0'),
(13, 1, 'system', '清理过期动态', 'cron_cleanfeed.php', 1416347948, 1416412800, -1, -1, 0, '0'),
(14, 1, 'system', '每日获取安全补丁', 'cron_checkpatch_daily.php', 1416353617, 1416421320, -1, -1, 2, '22'),
(15, 1, 'system', '定时发布主题', 'cron_publish_halfhourly.php', 1416357391, 1416358800, -1, -1, -1, '0 30'),
(16, 1, 'system', '每周广播归档', 'cron_follow_daily.php', 1416350694, 1416420000, -1, -1, 2, '0'),
(17, 1, 'system', '更新每日查看数', 'cron_todayviews_daily.php', 1416357389, 1416423600, -1, -1, 3, '0 5 10 15 20 25 30 35 40 45 50 55'),
(18, 0, 'system', '每日用户表优化', 'cron_member_optimize_daily.php', 1377225651, 0, -1, -1, 2, '0 5 10 15 20 25 30 35 40 45 50 55'),
(19, 0, 'user', '防水墙每日优化', 'cron_security_daily.php', 1416351485, 1416420000, -1, -1, 2, '0'),
(20, 1, 'system', '统计今日热帖', 'cron_todayheats_daily.php', 1416348808, 1416412800, -1, -1, 0, '0'),
(21, 0, 'user', '更新版块最后发表(防水墙相关)', 'cron_security_cleanup_lastpost.php', 0, 0, -1, -1, -1, '0');到此问题即可解决。