I deleted everything and reinstalled.
If create and save zoo-article without FB posting checked and after that reopen the article and check to post and save, THEN facebook post is ok.
If create+check to post the same time, Facebook posts a 404.
I checked our server's log and the server responds with 404 when create and check to post the same time.
Seems like the one-shot article-creation and FB posting, calls FB before the page is ready to be served by the server.
Also, your plugin does not respect the "Pending" status of ZOO-articles that are dated to be posted later.