error 404 The attempt to connect to the report server failed when trying to export report data

We recently added a cert to our report server and all seemed ok.

I am not sure how long this problem has been going on but now if we run a CRM report it works fine, but as soon as we take the "Export" (e.g. to Excel) option we get
"The attempt to connect to the report server failed. Check your connection information and that the report server is a compatible version
[WebException: The request failed with HTTP status 404: Not Found"

Other Info:

Request URL: https://OUR URL HERE/Reserved.ReportViewerWebControl.axd?........

When I look in the rsreportserver.config file the
<ReportServerUrl><UrlRoot> tags are empty. Is this normal?

Thanks for any help.

- Seamus