Permissions to use for Wordpress on Ubuntu with Webmin

You should be using a web group and user for this. Own the files to the web user and group and then change the permissions to something appropriate; 770 at the minimum.

Your apache process will be running as a web user of some type, find out what that user is and own the files to that user/group. Normally your web stuff is set 755 generally with specific permissions set on "special" or configuration files.

Do this out of hours and watch your http/php logs while you test out the configuration. There are standard permissions that the wordpress install will use. Look around and see if you can find them.

You could also install a new copy of Wordpress in a sub folder with the same config file as your current, get that running and then properly under a proper user/group and then move it into production after you have it working. This will save time and effort on changing your file permissions by hand.