This is a fairly common error. For the most common causes you will find you solution here .
Recently however I found that the article was not 100% the solution to my problem, but it reminded me to look in the Sft-server.log file.
In this instance the log file revealed that the server was still looking in the default content directory, rather than the new location specified during the installation of the server component.
I recalled the article on relocating the content directory. I found the key in the registry to be incorrectly configured. The key was not in the same location (this was a 2008 server, and it was App-V 4.5 SP1) but the principle was the same. Change the key(after a backup of course!), restart the streaming service and the issue disappeared.