A vaguely similar issue was posted earlier here.
What you can try is re-running the script with the -nocleanup option, and after the error occurs have a look at the image iter#_SF_diff.mif, where ‘#’ is the largest number that appears in the directory. For some reason, when mrstats is run on this file, no maximum value is being reported; hence Python is trying to convert ‘nothing’ (i.e. empty space) into an integer.
You can also look at the images iter#_SF.mif and iter(#-1)_SF.mif, since these are what is used to generate image iter#_SF_diff.mif.
If this doesn’t get you anywhere, you can send us a copy of the offending input file, and we’ll see if we can re-produce the fault.
Cheers
Rob