Trying to leave an edit summary gives me this error message:
Comment reserved for system use. Please use an appropriate comment.
This occurs both for anonymous and logged-in edits, for answers and for questions, using my phone (Chrome, Android) and computer (Chrome, Windows 11 Home). However, I can still perform logged-in edits, as long as I omit the edit summary. I am located in the Western United States.
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(IIRC) but I shouldn't have to do that when doing minor clean up of an antique question.