Hi Alan,
If you just copied private key file, then you didn't do things right. But before other dudes from community say something, pls read all :).
You need to backup folder keys and copy to new server or fresh installation.
But this is not only your "issue", was mine as well, site information about "how to move your tor server ..." are not up to date for some time now. Not so long, that method was ok, but last few 0.2.x versions migration don't work like before, and we can see much more files inside keys folder. That was not a case before, you could see only two files if i remember well.
I saw some people saying it's just bug and it will be fixed, but i don't like to see warnings either. In this time,or you copy all, or you wait for upgrade and disable arm for now.
Hope it helps.