The message shown by the plugin is in English instead of German (which is the wiki language).
I have solved this by myself: The language folder "de-informal" was missing (this is the German default setting for the Wiki language). One can rename the "de" folder or copy its contents to a new folder named "de-informal".
I recommend to update the plugin.
glreschke I recommend to update the plugin.
Maybe provide translations via https://translate.dokuwiki.org/?lang=de-informal
(don't know if the plugin in question is listed)
I have filled in and submitted the form (copied the contents 1:1 from "lang=de").