Hi,
Regarding converting .dcm to .nii, it seems to be a bug of that tool.
We recommend you to try to another tool to convert to nifti, like itksnap, or the simpleitk library installed for python. You can find an example of how it is done here:
persistent-shared-folder/APPLICATION-EXAMPLES/dcm2nifty.py
And you can use it with:
pyhton dcm2nifty.py seriesDir output.nii.gz
As for the second part (itksnap), it looks like a bug that they have already fixed in the next release (as stated here). So we have updated itksnap and found that it is now possible to open a Breast dicom file that previously could not be opened. To have the latest version of itksnap you will have to delete the desktop and recreate it.
Hope we could help!