Open IIS Manager, select your server on the last hand side, indicated by the * on the screenshot below.
Double-click URL Rewrite, shown in a red box in the screenshot below.
Double-click on the MirrorSync folder in the middle.
Click on 'View Server Variables'
The URL Rewrite dialog with MirrorSync and View Server Variables circled
If your server variables aren't set, add one with the name 'HTTP_IS_SSL' and value '{HTTPS}'. Be sure the checkbox for 'Replace the existing variable' is checked. Hit back to get to the last screen.
The Edit Inbound Rule dialog open with HTTP_IS_SSL circled
On the 'View Server Variables' screen, be sure the 'HTTP_IS_SSL' variable is allowed
