I am using a theme inherited from Antimatter. Once this theme is activated all generated pages start with an empty line. (I added line numbers to code below for better explanation)
I did some further testing and have created a simplified version of the website, i.e. child theme reduced to essentials, web pages just copied from antimatter example, most of the plugins deinstalled. Updates of today installed. Issue is still there. I have posted a zip of the simplified web site under the same link than the full child theme.