Occasionally, I'll have this problem with one of my machines. I delete the client.cfg file, and reconfigure the application, and everything runs great.
I suspect it refreshes the FAH server-of-choice that it pulls new projects from, goes to a different one, and works thereafter.
Been there, done that. On restart it gets assigned the same IP address to download a WU. I can ping it, and Stanford says it's running fine, but no go on the download.