'Parent Page' names not displaying correctly in 'Page Attributes' drop-down menu

Before I did two actions today, I was able to add or edit a Page and change the Parent Page by using the drop-down/up menu in the Page Attributes section. I just discovered that the list of choosable Page Names in the drop-down/up menu has changed, and many names are no longer visible (so not selectable). Also, one page at the bottom of my page hierarchy is viewable to the public, while another page at the same level is not, although I can find it through Dashboard. Also, the URL of the page in question was corrupted.

The two actions I took today that may have led to this state are: (a) changed the Navigation Label for two levels of sub-menu items on the main menu, and (b) invoked the website backup process. Any idea what might have caused the problem?