Try rebooting your phone/ipod/ipad. Stanza occasionally gives me that error, and if I reboot my phone and try again it fixes it. This sounds like a Stanza bug, and you should report it to the guys at
http://www.lexcycle.com/
Edit: I just reported my version of this problem
here. If that matches your failure, report that you're also seeing this problem. The more people report a problem, the more likely we'll see a fix in Stanza.