First, I'm running MacOS X 10.11.6 on a MacBook Pro (17-inch, Mid 2009).
Problem showed up today.
I create a new blank document in TextEdit.
I write what I need into the doc, then press ⌘+S to Save.
I name the file with (in this example) "test save.rtf"
When I click the <save> button, I get...