WordPress manages scheduled tasks with a system called WP-CRON. By default, this "tasks manager" sends two requests a day to update plugins or to publish scheduled posts. If there is no activity on the webserver, WordPress does not execute tasks or publish scheduled posts. In this case, the blog post stays unpublished while Blog2Social sends out the social media post as scheduled.
Facebook then displays the error message "Page not found" as WordPress has not published the post and Facebook cannot access the metadata and create the link preview.
The best way to avoid this error, is to have some activities on the blog. You can also set up a cronjob to send a request to the respective website every couple of minutes.
Further information on WP-CRON: https://code.tutsplus.com/articles/insights-into-wp-cron-an-introduction-to-scheduling-tasks-in-wordpress--wp-23119