german Umlaute not rendered in html

German Umlaute in author list on index.html are not rendered in the html.

Connected to authorlist in sphinxext?

authorlist is a directive from hereon_sphinx_ext: link to code in extension

authorlist seems to be unable to render Umlaute, where do I set the charset in sphinx? Is the reason a YAML or cff compilation? Or is it also connected to the Sphinx extension above?