The queue database is probably corrupt.
What you can do is as follows:
1) Close Usenapp
2) Open Finder
3) In Finder press SHIFT-CMD-G (or in menu Go->Go to Folder)
4) Then in the text field enter the following text (including the tilde ~) and press “Go”
~/Library/Application Support/Usenapp
5) Select the “queue” folder and delete it.
6) Start Usenapp