Jan,

The issue is one of two:
(1) The location of the files in SPM.mat no longer exists (e.g. SPM.xY.P
files were moved);
(2) The ROI is outside of the brain mask for this subject.

If #2 is the source of the error, you can either (a) mask the ROI by the
group mask; or (b) set P.FLmask to be equal to 1 instead of 0.

Hope this helps.

-Donald