Kristof Schmit on "[Plugin: Contact Form 7] Checkbox content is no longer submitted in email output"

By the way: I have never before recognized the HTML output of the form:

<input type="checkbox" name="Gutueberdacht[]" value="Vordächer">

I wonder if the [] in the name field is the cause of the issue? But where does it originate from?

Thanks for your help!