CCILandCover-product contains no data

I´m running step 4 of the SEN-ET Plugin (“Add landcover graph”) but the product contains no data (the band land_cover_CCILANDCover-2015 ist totally empty/dark). This appears with different Sentinel 2 scenes. I use SNAP 8 with Python 3.6 at Win 10. Neverthelees I don´t get an error message. Does anyone have a suggestion, to solve this?
Regards, Rainer