Very strange.
I just did a test on Mojave and all works fine.
Maybe it helps resetting the preferences.
1) Close Usenapp
2) Open Finder
3) In Finder go to Applications/Utilities and open Terminal.app
4) Type the following command in Terminal.app and press enter:
defaults delete com.xoroxsoft.usenapp [enter]
5) Start Usenapp.
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