The queue database is probably corrupt.
What you can do is as follows:
1) Close Usenapp
2) Open Finder
3) In Finder, press Shift + Command + G (or use the 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