Alas, I’m not able to replicate this one here on any of our in-house test servers and/or clients. And to date, no one else has reported a similar issue that I am aware of. It doesn’t sound like an issue with WHS RemoteApp, but more of an issue with the underlying network connection from the client to your server (hence, restarting its Provider service will not make any difference).

Does it happen on all of your client computers, or just the one?

Are you certain that you have all of the latest Windows Updates installed on both your server and your client computers?

Is this something that only just recently started occurring, or has it been an issue for you since you very first started using WHS RemoteApp? If it only recently started happening, then do you recall doing anything to the server/client, or to your local network, router, firewall, etc. that may have triggered it? Have you tried restoring your server (from server backup) back to a point in time before you first started seeing the issue?

Lastly, that particular error message will be shown to the user (by the underlying Remote Desktop Connection functionality of the client computer, and not by WHS RemoteApp) if the client computer is unable to locate a valid IP address for the server. Is your server using a dynamically assigned private IP address (which is the default configuration in WHS 2011)? If so, then I’d try configuring static IP addressing on the server for more consistent results.