Looking at this, it appears the old problem that's been going around for ages and has not been fixed, despite many people bringing it up.

Forums that are configured as, "not visible" to guests. The widget should ignore but instead, posts are being listed in recent posts widget and showing as 53-years ago.

Before this recent update, it would just list the post title and 53-years ago, now it's giving this new warning message.

As you can see from the attachment above, guest visible posts are showing without error.

Why do I always regret doing a wpforo update.

Fixed by applying this to functions-template.php

https://wpforo.com/community/how-to-and-troubleshooting-2/empty-items-in-recent-posts-widget/#post-49012