Hi, I fixed it:
In FacebookDeveloper App Settings Advanced I chosed “Reset client token”
Than in Facebook Login Settings, menu Login Connect with Messenger
in the Add Page I put the page where I want to post (it asks the PageID , which is in About menu of the Facebook page). But I left the “Login Connect with Messenger” Off.
After a Reauthorise in WP, the page appeared in the list at the bottom and publishing is ok again.
Best regards,
Mihnea