Thanks for answering.

I think, you have a bug in your theme.

Although I had inserted an excerpt, the blog listing page shows the full text of the article.

Can you fix this issue, please?
If not, what can I do to display the excerpt text?

Blog view:
https://www.awesomescreenshot.com/image/3957028/d4af8672358b7604c0d8c22c05d299f2

Article excerpt:
https://www.awesomescreenshot.com/image/3957033/73072cd19ad3b9c35bd06edf939a4d98

Best regards
Mathias