There is an annoying bug in the 5 minute edit period. For a minute or two after the 5 minutes it will allow you to open an editor, but won’t allow you to save your changes. This has sometimes been a source of annoyance to me. Often I’ve noticed a spelling mistake or other minor error in my post. I’ve hit edit, made a change which takes maybe 5 seconds, but can’t save it. It would be less frustrating if it didn’t open the editor at all. Any chance of doing something about this?
You might find some answers in this thread.
The thread starts off on a different topic, but gets into the situation you described.