I found the source of the error.
In the script of the the cut stimulation process "process_cutstim.m", line 104, the variable containing the name of the interpolation method is expected to be a cell containing a string. Instead, in the script of the "Cut stimulation artifact" process generated by the option "Generate .m script", a string is associated with the option 'method', and not a cell containing a string (line 15 of the script in attached).
Hence the mismatch and error. Writing {'spline'} instead on 'spline' as the 'method' option of the bst_process function fixes the issue.