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Just to clarify - do you manually edit the URL (slug) press the button "update" but the URL slug is not saving but reverting back to its prior (default) value? If that is the case, there must be something wrong with URL (not allowed symbols) or some plugin issue messing it up. I would try to disable plugins one by one and see if any of them is causing the issue.

Few things more I'd try to troubleshoot the issue:

• Trying to do edit slug using "quick edit" and "edit"
• See if the same issue is for all posts or just one specific post

Also, the slug will not update until you click Update button on the Edit Post screen. If you click on Permalink then edit, that will allow you to update the slug, then hit OK, but when you click on OK that will not update the permalink.