This is no longer an issue since version 2.0 and the rewrite of the macro in Java (PLANTUML-5, https://github.com/xwiki-contrib/macro-plantuml/commit/0602f87): the PlantUML server is now called by XWiki server-side and the generated image is served by XWiki as a temporary resource, so the browser never gets an image link built from the configured server URL.
In addition, since 2.4.1 (PLANTUML-25, https://github.com/xwiki-contrib/macro-plantuml/commit/c8b19bd) a relative server URL such as "/plantuml/" is rejected with an explicit "Invalid PlantUML Server URL" error. Use an absolute URL instead (e.g. "http://localhost:8080/plantuml/" if the PlantUML server runs on the same host).
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