403 forbidden error when loading images in test server
This is a typical result of an incorrect url path.

That folder may/may not exist on the test server, but because it's outside of your user's folders, you 'don't have permission to access' it.

For example: Typical user's folders for cPanel are /home/USER/ with the site root at /home/USER/public_html. But, typical for a local machine is /var/www/....

So when you call sites/default/... from a local machine, it is possible that it's actually looking at /var/www/sites/default/.... When doing that on the test server, the path would be outside your user's folder and therefor no access permission.

It could be as simple as a missing ../ on your relative paths.
So you could try 1. adding ../ to the start of your paths and see if that takes it to the correct location.
or 2. Try explicitly setting the base url in your settings file $base_url = 'http://foobar.com'; to force a starting point for relative paths.