If a backups fail when using the built in backup feature off moodle.  Check your apache error log.  I was getting ‘ALERT – configured POST variable limit exceeded – dropped variable ‘backup_user_info_resource_instance_25051’

Edit your PHP Suhosin module configuraion either withing php.ini or on SuSe Enterprise Linux /etc/php5/conf.d/suhosin.ini

Remove the ; and increase the value e.g. from:
;suhosin.request.max_vars = 200
;suhosin.post.max_vars = 200

suhosin.request.max_vars = 400
suhosin.post.max_vars = 400

Then stop and start apache for the changes to take effect. apache2ctl stop then apache2ctl start.