Dear mrtrix community,
Sorry for answering so late - another topic and the clinical use test of a major new software at my university grabbed my attention completely. My sincere apologies.
Coming back to your answers I want to ask you for the correct syntax and inputs for the continue option. The last file I generate with the
dwipreproc -force -pe_dir AP APPA_denoised_unrung.mif APPA_denoised_unrung_preprocessed.mif -rpe_all -se_epi b0_AP_PA.mif -eddy_options "--niter=8 --fwhm=10,6,4,2,0,0,0,0 --repol --ol_type=both --mporder=8 --s2v_niter=8 --slspec=slspec_AP.txt"
command is field_deriv_pe_1.mif as invoked by
[...]
dwipreproc: Detected matching DWI volumes with opposing phase encoding; performing explicit volume recombination
Command: mrcalc field_map.nii.gz 0.0681 -mult -1.0 -mult - | mrfilter - gradient - | mrconvert - field_deriv_pe_1.mif -coord 3 1 -axes 0,1,2
dwipreproc:
dwipreproc: [ERROR] Command failed: mrcalc field_map.nii.gz 0.0681 -mult -1.0 -mult - | mrfilter - gradient - | mrconvert - field_deriv_pe_1.mif -coord 3 1 -axes 0,1,2 (dwipreproc:996)
dwipreproc: Output of failed command:
mrconvert: [ERROR] omitted axis "4" has dimension greater than 1
I then followed the suggestion given by @jdtournier and extracted the first volume of field_map.nii.gz as described by him.
However
dwipreproc -force -pe_dir AP APPA_denoised_unrung.mif APPA_denoised_unrung_preprocessed.mif -rpe_all -se_epi b0_AP_PA.mif -eddy_options "--niter=8 --fwhm=10,6,4,2,0,0,0,0 --repol --ol_type=both --mporder=8 --s2v_niter=8 --slspec=slspec_AP.txt" -continue dwipreproc-tmp-1YXW7U field_deriv_pe_1.mif
and
dwipreproc -force -pe_dir AP APPA_denoised_unrung.mif APPA_denoised_unrung_preprocessed.mif -rpe_all -se_epi b0_AP_PA.mif -eddy_options "--niter=8 --fwhm=10,6,4,2,0,0,0,0 --repol --ol_type=both --mporder=8 --s2v_niter=8 --slspec=slspec_AP.txt" -continue "dwipreproc-tmp-1YXW7U" "field_deriv_pe_1.mif
and
dwipreproc -force -pe_dir AP APPA_denoised_unrung.mif APPA_denoised_unrung_preprocessed.mif -rpe_all -se_epi b0_AP_PA.mif -eddy_options "--niter=8 --fwhm=10,6,4,2,0,0,0,0 --repol --ol_type=both --mporder=8 --s2v_niter=8 --slspec=slspec_AP.txt" -continue "dwipreproc-tmp-1YXW7U" "dwi_post_eddy.nii.gz"
all produce the same error message:
mrinfo: [ERROR] failed to open key/value file "dwi.mif": No such file or directory
mrinfo: [ERROR] error opening image "dwi.mif"
dwipreproc: [ERROR] Could not access header information for image 'dwi.mif'
I am very grateful for your assistance 
On a the good side I can give you the feedback that if your workflow is ok with the Siemens Magnetom Vida XA11A enhanced DICOMs it will be so too with XA11B.
The important point is to always have selected enhanced DICOM (in the total chain of storage, i.e. if you are getting them directly from the scanner as well as if you push it via the PACS before exporting)