Hello A., F., and all,
Here is the link: https://plantuml.com/guide
The fact (or the defect) is the redirection between https to http (with the redirection from https://plantuml.com/guide to http://plantuml.com/guide)...
See also: https://forum.plantuml.net/12909/plantuml-language-reference-guide-http-vs-https
if that can help,Regards,Th.
(--------)