Issue: Book editing error

I cannot reproduce this in any of the following cases:
- Edit Book > Preview
- Edit Chapter of a Book > Preview
- Create Book > Preview
- Create Chapter of a Book > Preview
- Create Book as Chapter of a Different Book > Preview

If you can reproduce this issue consistently on different books, please provide more information.

After reviewing the cause of your other report, I have determined that this will occur as another side effect of the same bug. Marked as a Duplicate: https://www.vaultwiki.org/issues/4799/

In fact, this error is shown by an orphaned page. previously removed from a book chapter list.
It cannot be edited or previewed, until it will be assigned to a book, no matter if it's the old one or new one.
Other way to restore this bugged page is to delete it, and re-create it with the same content.

yes it's related to book tags.

This issue is not resolved yet. An example is http://www.projectetal.com/forums/wiki/?title=Slack_com

I suggested a couple of months ago to @Ethic to resolve this book tag issues, and after that to delete the remaining 2 books, and to prevent creating books from admin panel. For that site "book" tag is useless anyway.
I'm still remembering the first wiki crash cause the most of pages were at that time with book tag checked on.