I would carefully heed the warning whenever you run dcm2nii that you should use dcm2niix. dcm2nii is deprecated and will not be included in future releases. I would only recommend it for archival studies that pre-date DICOM (e.g. proprietary Elscint, Siemens and GE image formats). The vendors are continuously evolving their implementation of DICOM, so you need to continuously upgrade your DICOM software. If you have issues, file an issue on github   https://github.com/rordenlab/dcm2niix/issues
Since I do not have access to PET images, we are necessarily dependent on samples from the community.