I'm trying to edit a script in ~/Library/Scripts/Applications/Mail. When I attempt to save from Script Editor I get The document "Reply.scpt" could not be saved. (Error -1,752). It doesn't seem to be a permissions issue because I can edit and save the file in BBEdit (it's a plain text file).
I'm running Script Editor 2.8.1 under OS X 10.11.1 on a late 2015 iMac.