I am trying to take a Backup of My server from the admin panel . Using following options

System >> Tools >> Backups >> System Backup

It gives the popup to mention the name of the file of the backup and gives a loader for some time . Then it gives me a 500 internal error.

The error stays untill i change the permission of the index.php to 655 .

I request you please provide me the reason behind this error and solution for the same.

Thanks in advance.