Which version of DNG Converter are you using? The latest available one is 6.7. Please download and try that if you're using an earlier version.
I had the same problem. My 2 cent:
(1) Reboot your computer [I work in I.T., this usually fixes all the problems users are experiencing]
(2) Make sure you have space in your destination/source directory where the images are going to be added.
This is what happened to me: I selected my memory card where all the (RAW) images are located. My "save to" location happened to also be my memory card. Basically, I had no more space on my memory card, but I was asking Adobe DNC to convert and save/add those images into the same location, my memory card. Which gave me the exact error message you are reporting.
Fix: I just created a new destiatoin/location on my computer's hard drive where Adobe DNC can save those converted images. Works for me; I hope it works for you.
Nice of you to offer tips, visitor4rmmars.
The post you're replying to, however, is over one year old, and the OP says he did in fact reboot his machine.
Your 2 cents solved my problem. THANK YOU so much for replying!