Today someone told me whatever he do,no matter what but cronjobs is not working
I got access details and open ~cron.php manually from browser,I got 500 error,So I turn on debug modes to see what's going on !
I found out emails causing this problem So I went to database(base_user table),change the email and everything's working fine.
the bug is,Oxwall validation for emails is not good,you should look for a better regex
in 6000 users 2 users email were like this
username.@example.com
the dot before @ raised an error in cron