403 Forbidden response – what should I look for?

Check the permissions and also ownership of the file. Generally, 403 means that the web server doesn't have the rights to read the file and therefore can't continue the request. The permissions may be set correctly, however the file might be owned by another account on the server - an account that isn't part of the same group as the account which is running the server.

For instance, I believe* Apache is ran by default under the httpd user account, which is part of the httpd group. However, the FTP user you're logging in as (for instance ftpuser) might not be part of the httpd group. So, in copying the file you've created it under a different user account and Apache won't get execute access with 644.

* it's been a while since I've used apache, but it's similar under nginx.

This isssue occurs if you have had denied for all in .htaccess file. Changing that resolves the issue.