That's the most likely cause.I assume you cannot load it into Slicer either.

The DWIConverter module uses a program called DWIConvert, which part of the BRAINSTools project, and you contact the corresponding developers:
http://wiki.slicer.org/slicerWiki/index....
https://github.com/BRAINSia/BRAINSTools/...

Alternatively you could convert with FSL (if that works) and then DWIConverter to convert from NIFTI (nii.gz) to NRRD

Best regards
Martin