Same issue when classifying LANDSAT images. After several trials though I found out that all classifications work when the image is reprojected to WGS84 geodetic reference system (lat,lon). So it must be an issue due to wrong handling of the reference system
Running SNAP 5.0.7 (fully updated) on linux
Another minor issue on linux is that the maximum likelihood classifier searches for a file MaximumlikelihoodClassifierGraph.xml in /.snap/raphs/internal/classification/, but the existing file is MaximumLikelihoodClassifierGraph.xml (L uppercase instead of l lowercase)