Fixed it. Found this solution:
The problem was a corrupted settings entry in the Windows registry. When the following hex value was deleted, functionality was restored:
HKEY_CURRENT_USER\Software\Adobe\DNG Converter\8.0\Preferences
Presumably the registry entry got corrupted somehow, but the DNG Converter software should not have been affected in this way. The software should be able to detect and repair/replace corrupted settings.