Issue: Can't save Book-text

It seems you have a lot of files present that should have been removed by upgrades. If the upgrade script wasn't able to remove these files automatically (possibly file permissions related), then they must be removed manually. After looking into your case, it seems you have files present from as far back as Gamma 2 that should have been removed by now.

I would recommend starting here: https://www.vaultwiki.org/pages/Book...-4-0-0-Gamma-4

Look at the "Files" section for "Any" and "XenForo" and make sure those files are NOT present on your server. If they are, then they need to be deleted from the server or they might interfere with proper operation of VaultWiki (as you noticed you can't save changes to text for book pages - that is a common symptom of this).

When you are done with that list ("Changes in Gamma 4"), proceed to the next page ("Changes in Gamma 5"), and repeat the process until you reach your current installed version and all the undesirable files are gone. Some pages won't have any Files to remove, so just keep going. Some of the files listed will be image files -- you can just ignore those.

I am confident that this will resolve the book-text issue.

Ok, now it works!