I had the same for matlab 2018, so it is not caused by version.
I later managed to overcome it by carefully checking if the output from SNAP was complete and then using the updated scripts provided by Katherine (see the overview post here: About the STaMPS category)
It is also a good advice to run StaMPS in matlab only step by step and check if all results were written correctly because otherwise they are missing in the subsequent steps without major warning.
Run single steps like this: stamps(1,1), check results, stamps(2,2), and so on.