If I use an hard return in the textblock, everything after does not get shown on the page.
I have to use soft returns (shift+return) to achieve a whitespace.
Why?
I get several complains about this situation, and I am not sure how to solve this.
So you are using a default text block (nothing changed in your text.yml)? If you have a custom yml, please post it here. I cannot reproduce your issue. And you output the content with the code above? The resulting html will be invalid, btw, because you can only have phrasing content in a span element.