If Essentials is behind a reverse proxy, it will not make the correct URLs to get template reports (printed reports).
To resolve this you must set the the External REST URL in the Application Settings to be an appropriate address to Essentials. If Essentials is in a named instance, then make sure to include the correct path - the suggested one will be wrong.
For more information, refer to the Essentials documentation topic about running Essentials behind a reverse proxy.
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