I've seen this happen a couple times, and don't know what it is. I have only seen it when using a networked filesystem, so it is possible that there is something that can happen there -- a timeout when the filesystem is busy?

The program can proceed because the minmax.1D files are a side-calculation to compute the single-voxel threshold that would give a 5% (say) false positive rate, globally. I only put this feature in there because someone (whose initials are TN) asked for this -- and as far as I know, never used it. If it fails to read the minmax.1D files, it just skips this calculation and goes on to the main Clustsim work.

To be clear, the threshold computed this way (if it had actually worked) would be the z-statistic to threshold per-voxel such that there would be a 5% chance of ANY false positive voxel inside the brain mask. This value is somewhat below that which you would get from applying the Bonferroni correction -- choosing a per-voxel z threshold such that the corresponding per-voxel p-value is 0.05/Number_of_brain_voxels. The threshold this way is still very strict, and so probably not useful unless you really ARE interested in single voxel results -- and have really good data.