I have the same issue. The github website below does not have a Office365 specific template file-
https://github.com/plantuml/word-template
I tried downloading and putting the Word 2016 template file in the STARTUP folder. But Word is not able to open it, or when I open Word, it gives me error that it cannot open the .dotm file.
Do we need a specific template for Word Office365 ProPlus?
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