error in pre-processing step after flipping left/right hemispheres

Hello everyone,
I am facing the following error in the pre-processing step in CONN version 17.f. I would appreciate if you let me know why is that and how I can fix it.
a background about what I have done is:  I have 9 subjects , with pre/post sessions.  one of the subjects has only pre, and doesn't have post. so I checked the "allow missing data" box. In the preprocessing first I flipped left and right hemispheres for functional and structural images by choosing none rigid reflection along x-axis, and it worked with no issues and flipped the images.
After that, I went through preprocessing again, and this time I selected default pre-processing, and after a while it gives me the following error. I also got many warnings in the Matlab Command Window including: "Warning: Matrix is singular to working precision". , and " Warning: Forcing deletion of MAT-file"


ERROR DESCRIPTION:

Error using spm_jobman>fill_run_job (line 461)
No executable modules, but still unresolved dependencies or incomplete module inputs.
Error in spm_jobman (line 247)
sts = fill_run_job('run', cjob, varargin{3:end});
Error in conn_setup_preproc (line 1940)
job_id=spm_jobman('run',matlabbatch);
Error in conn (line 894)
ok=conn_setup_preproc('',varargin{2:end});
Error in conn_menumanager (line 120)
feval(CONN_MM.MENU{n0}.callback{n1}{1},CONN_MM.MENU{n0}.callback{n1}{2:end});
CONN v.17.f
SPM12 + DEM FieldMap MEEGtools
Matlab v.2016b
storage: 1116.6Gb available
spm @ E:\NEUROSCIENCE\spm12\conn17f\conn
conn @ E:\NEUROSCIENCE\spm12\conn17f\conn


also, the last line in the Matlab command when I get the above error is:  "Item 'Session', field 'val': Number of matching files (1) less than required (2)."
P.S. I am not getting error when I simply use the pre-processing, without first flipping left and right hemispheres.

Any help will be highly appreciated.