Anything that could affect remote connecting for your site, e.g. socket_set_timeout or stream_set_timeout. If that is set to 0, you need to change it to the default of 90 seconds.

Look in .htaccess, php.ini or .user.ini files.

Also, the fact that you got a 403 error for that test.php file is really weird. It should have just output if it works or not...