SPM.mat wasn't attached.

The necessity of a contrast image depends on the settings. If you set P.contrast=1, then you do need a contrast image. In this case, it should have given you error message that the inputs aren't valid. I would advise against setting P.contrast=1; rather, set P.contrast={'Omnibus F-test for PPI Analyses'}. This is better for several reasons:
(1) It will use the same contrast for all subjects, using a number is bad in case you create an extra contrast in some subjects;
(2) It is an F-contrast to remove noise from the data before deconvolution;
(3) If the contrast does not exist, it will create the contrast.

It should also generate an error if the contrast is not created. If the contrast is not created, then it indicates a problem with the directory and file permissions.

Let me know if changing P.contrast fixes the issue with this subject.

Best,
Donald