I had two calls on this one today! I expect more.
When you enter ED at the command line and hit enter you get "Unknown Command" in AutoCAD 2015. It still shows you the DDEDIT in the "AutoComplete" fly out though.
Run the ALIASEDIT command (or edit your PGP file) and change the command associated with ED from DDEDIT to TEXTEDIT.
It seems this was a change that was made in 2015 as it still works in 2014!