With try&error I found it ;-)
"Limit frontpage text" limits also the text of blog postings on the "blog" page.
But what I've discovered:
with this settings, so that the posting text is limited to (in my case) 80, the formatting of the text excerpt has been gone: every new line, empty lines between paragraphs, bold ... has disappeared in the excerpt on the blog page.
But when opening the postings, the lines and all other formatting is again there and o.k.
Why does not the excerpts show formatted text, new and blank lines between paragraphs for example?