After you create/load/modify P, can you use the command:
[P, errorval]=PPPI_checkstruct(P)

To stop the program, you can insert the word keyboard on the line before
you want the program to stop. This way, you can stop right before the PPPI
line and run the above command instead. This should produce a more detailed
message about why the program is failing.

If you get an error about Contrasts field not existing, then the likely
source is that defContrasts.m isn't in the matlab path.

If you type:
which defContrasts.m

What is the output?

Once you have added it to the path, then everything should work.

Hope this helps.