This is a known issue which you may have seen once you uploaded files using the SFTP service. The problem is caused by some SFTP clients which enforce (the wrong) file permissions making the file unreadable by the webserver. This issue can also occur when you create files using PHP.
The fast and easy solution for such a problem is to run a "Fix File Permissions" operation. This can be done from within the Control Panel or from within the Pressidium WordPress plugin which is installed in all your websites. You can read a detailed description on how to reset file permissions for your website here.
We've recently released a service which acts like a file permission watcher and once it finds a file with wrong permissions it immediately corrects them. This means that you should never encounter this issue again!
We won't remove the "Fix File Permission" operation yet, since this is a relatively new service and we also need it for backwards compatibility.
What do I need to do ?
Most probably nothing, the file permission watcher service will do it for you, however since it's a relatively new service we've also left the Fix File Permission operation intact.
Bottom line, if you see a 403 Forbidden permission error your first reaction should be the Fix File Permission operation. If this operation doesn't resolve your issue, please create a support ticket and we'll help you.