Change DB Prefix

Hello,

sorry, but there maybe another permission-issue, which I cannot resolve in the moment.

I tried to change the DB Prefix and while I receive the confirmation

“Starting DB prefix change operations…..
Your WordPress system has a total of 45 tables and your new DB prefix will be: …_wp”

there’s a warning

“Warning: copy(/www/htdocs/w00c71b7/apropos/wp-config.php.backup) [function.copy]: failed to open stream: Permission denied in /www/htdocs/…/…/wp-content/plugins/all-in-one-wp-security-and-firewall/classes/wp-security-utility-file.php on line 49”

and the message “Failed to make a backup of the wp-config.php file. This operation will not go ahead.” and the Prefix still remains wp_.
So, which file I have to change permissions?

http://wordpress.org/plugins/all-in-one-wp-security-and-firewall/