Which matlab version are you using?
The issue may occur if on the step 1 you had not use the matlab script that filters the data when lat/lon coordinates are not valid, taking values or NaN or 0 generated by SNAP.
The solution was already provided by me on the forum, for which you need to update/overwrite the ps_load_initial_gamma.m script on the stamps/matlab folder
Here the link: New StaMPS release (4.1-beta) - #27 by mdelgado