This is an issue that had me banging my head for far too long. Whenever I went into my FAST Query SSA, clicked on FAST Search Administration, clicked on Managed properties, and then either Add a new managed property or Edit an existing one I was presented with an "Unexpected error occurred while communicating with Administration Service" message. After searching the web I came across this great blog post by Gavin McKay which has a few things to check to make sure that your FAST for SharePoint server is setup correctly. After running through the list and verifing that I was good there I was still getting the error.
What it came down to was the path statement on my FAST server. You need to make sure that the \FASTSearch\bin directory is the first entry on your path statement. On my server it was the 7th entry in and as soon as I changed it to be the first and then rebooted, I was back in business.