I searched the DNG Converter threads first, didn't see a question quite like mine.
The SD card in my D3200 "lost" about 150 files yesterday. I downloaded several "freeware" file recovery apps, mostly they didn't work well, though they did install other software without asking. And these were from the CNET site! I finally found one that worked perfectly (not on CNET), Stellar Phoenix Photo Recovery, then paid $39 to get a product key so I could copy the recovered files from the SD card to my convert folder on my internal drive.
I have an early Mac Pro with Lion 10.7.5. I also still use Photoshop CS5.1. I have been using DNG Converter 188.8.131.52 for several years just fine, but when it failed on the recovered NEFs, I installed 184.108.40.2061. It fails too.
DNG Converter gives this error on the recovered NEF files: "The source folder does not contain any supported camera raw files." It still works just fine on NEF files copied from the SD card that were never "lost".
Additional note: I just opened both normal and "converted" NEF files in a text editor to compare them. The "recovered" files have no recognizable header. My basically uninformed attempt to paste a header from a good file onto one of the recovered files accomplished nothing. (I sometimes had success with that back in the early 1990s...) I've also sent a query to the recovery software company. Maybe I've just learned a lesson about the life expectancy of SD cards?