Really can't thank you enough for your brainpower on this. 

Here's what I got: 

K>> [P, errorval]=PPPI_checkstruct(P)
??? Error using ==> save
Unable to write file /autofs/space/lobulus_002/users/NEFS_aging/B12871-ef/nifti/2back/2back_motionmodel/invalidPPIstructure.mat: permission denied.

Error in ==> PPPI_checkstruct at 711
save([pwd filesep 'invalidPPIstructure.mat'],'P');