When moving a page with 5 or more children to a new space, the UI component shows "Move failed. There was a problem contacting the server." The page cannot be moved.

A related article from the knowledge base originally suggested a data integrity problem with the Ancestors table, but there is a replicable bug as well:

Steps to reproduce:

1. created a page w/ no page content. Attached 2 .png files to the page
2. created 5 copies of this page and then, using the Space Admin Pages view, drag & dropped them into a hierarchy - one page below the other, until the pages are a total of 6 levels deep - 1 child page below each parent
3. use the Tools -> Move Page menu item. Select another Space and tried moving under the Home page for that Space

Result:

1. get "Move failed. There was a problem contacting the server." in the GUI
2. ALL attachments to all pages in the page tree are gone – seemingly irrecoverably (short of going to backup).

Workaround
There's a recovery script posted here. There's a find and recover attachments script there.