Discriminant Analysis dialog box
Since Box's M is significant, it's worth running a second analysis to see whether using a separate-groups covariance matrix changes the classification.

step  To obtain a classification using a separate-groups covariance matrix, recall the Discriminant Analysis dialog box.

step  Click Classify.

step  Select Separate-groups.

Note that with separate groups, leave-one-out classification is not available.

step  Click Continue.

step  Click OK in the Discriminant Analysis dialog box.

The classification results have not changed much, so it's probably not worth using separate covariance matrices. Box's M can be overly sensitive to large data files, which is likely what happened here.