It seems that it is not possible to (re) publish posts from list view in backend, if the article has a date for unpublishing. So I have to go to the post entry and delete the unpublishing date. Only then I can republish the article.

This is very uncomfortable if you have some hundreds of articles to republish ...

(If you try to re-publish from list view eb answers that the article(s) are published, but in fact they are not published)