As a followup to my previous post, I was trying to create an Apple Remote Desktop installer. This is usually something that's fairly trivial:
Create Client Installer...
However, on trying this on my 10.11 Server install, I was instead presented with this:
In theory, ARD is supposed to automatically download components (like the client) which it doesn't have, and I actually managed to coax my local install of ARD, which had previously been giving the same error message, to do so.
In any event, if this happens, you can actually cheat and grab the client yourself from Apple's servers:
Not really sure if this is a good idea or not, but it fixed the problem for me!