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Adjust the PlantUML for Confluence app settings to use your externally accessible, HTTPS-secured server by configuring the Custom URL in the apps settings, found under:
Confluence Administration → PlantUML for Confluence

Type of Diagram Files:

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For PNG diagrams: https://domain.to.server.com/plantuml/png/

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For more information, checkout Custom URL for diagram generation - PlantUML for Confluence

Source Code and Further Information

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