You receive that error message because the file you want to open seems to be corrupted. In order to fix the problems, I recommend that you try opening the .dbf file using DBF Manager or DBF Viewer. If you can open it, launch S-Distance and choose another method to import the database. Select Import Database from the File menu and then check either Network Coverage or Point Coverage option:
If you still have problems, try updating the program to the latest version. Also, you can contact the developers using the contact information from the official web page.