It appears Apple changed something in the iOS bundle upload : it now requires that all executables in your app bundle are signed (this might be applicable for Mac uploads too, but I haven't tried). I hear you think: isn't this the case anyway, but there might be more executables in your bundle than you expect.
Of course, there's the binary for your app, which obviously is executable. But there could be more too: if you include shell scripts, for example, which are marked executable (
chmod u+x script.sh) then iTunes Connect now considers them as actual executables and now requires them to be codesigned.