Error 403 when downloading image

Hi dev,

I am using Instant Articles for WP to create an instant article out of existing posts. However, the images cannot be downloaded by the plugin because of an 403 error code, even though the URL links to the image.
Here is an example of the error:

Could Not Download Image: Your image from URL https://www.theincentral.com/wp-content/uploads/2016/07/o4e7c7e35smlshmqlzwa.jpg could not be downloaded with an error code 403. Please check to make sure image at https://www.theincentral.com/wp-content/uploads/2016/07/o4e7c7e35smlshmqlzwa.jpg is active and loads correctly at that URL.

The image is accessible, but somehow the Plugin cannot download them to add them to the Facebook instant articles. My upload directory has 0755 permission and the images all have a 0644 permission. I am using Cloudflare as a CDN and W3TC.

Has anyone experienced this Error before and could help me out?
Thank you