The Amazing Replicating Page!
When attempting to edit pages with UTF-8 characters in the title, when UTF-8 support is not enabled, and a URL Replacement usually handles the character, an order of operations issue causes an improperly encoded string to be tested against the database of existing articles. Additionally, a problem in the templates where the forumid is sent with the form during non-create actions treats the new title as a new article, rather than updating the old title.
Fixed in the current ZIP. If you are experiencing problems with renaming pages, you can fix your templates by running vault/install/finalupgrade.php again.