@PanosMan a few checks that might help.
Calculate statistics on the dataset then reapply symbology.
What inputs produced the output raster? 2 different rasters? Are they in the same coordinate system and raster type? (8bit unsigned)? Do the pixels overlap? Try using snap raster to realignt he input rasters before running raster calculator.
Try working with .tif format instead of whatever format you're using.
David
..Maps with no limits..