The scenarios that can cause a page to throw 404 upon editing are numerous and I might not be correct in pinpointing the exact cause and I help the scenario I describe here will help in terms of checking if the problem is coming from the page url.
If upon editing the page the url is different from what it should be when editing a page, site.com/mypage/action/edit
. The url construction is incorrect.
This can be caused by the parent pages of this page. Most probably the page is located under another page or a set of pages. To make sure if the parent page(s) cause the problem temporary move the problematic page to be first level page (not under a parent page) and see if its editable now. If its editable then one of its parent pages have an additional url (select Actions->Title and Properties for particular page to see additional urls) and test removing additional urls.
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