This article describes an issue with A/B Testing after setting up Azure IP Restrictions. The error 403 occurs for A/B testing thumbnail images because the requests for the thumbnails are made from the server on which the site is running.
In Azure, you can add IP rules that allow/deny traffic to your site or app. When you create a single rule for access, everything else is denied. So, if you set rules to allow/protect access to your site, add a rule to allow access for the IP address of the server that is hosting the site. This will resolve the 403 error.