Typically this error occurs when there is an issue with the routing rule, causing incoming traffic to not appropriately be sent to your backend pool.
Make sure that you have a routing rule that works for HTTPS, as well as making sure you have a separate routing rule if you have a custom domain on your front door.
If your environment is HTTPS only and you do not have a HTTP to HTTPS redirect, make sure that you are using HTTPS when you connect to the front door!