Great. The validation datasets demonstrate that the latest stable release extracts slice times from a CimaX running XA61 for both the product and CMRR research sequences:
  https://github.com/neurolabusc/dcm_qa_xa...
Therefore, if you do not have slice times, my first guess is that critical DICOM tags were stripped from your dataset prior to dcm2niix (persumably from an anonymization stage). You can test this by downloading images directly from the scanner console to a USB device, without having a PACS manipulate the images. If you still have an error, upload the JSON that dcm2niix creates. This is also a great opportunity to work with the Siemens Research Collaboration Manager associated with your center.