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.

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

Click Classify.

Select Separate-groups.

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

Click Continue.

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.