Got frustrated several times that would not accept my change.
Created some tests and discovered was |. Easily reproducible... add attachment --> enter url of "example.com" --> rename it to "Some Title | Some Site" for example --> try to change that.
unfortunately this is a common format many sites use (e.g. "HTMLInputElement - Web APIs | MDN" if dragging in https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputElement)