I'd suggest to fill it all the way with as much salt as it would take without over-flowing. Then try different hard water settings from low 0-4 grains up to 20+ grains and see if the "add salt" light still comes on. If it does, then I'd say have a Miele tech come out, or maybe they can tell you a way to reset it over the phone. I know in the manual (near the end) there is a way to do a full factory reset of the dishwasher that sets the defaults for the rinse aid and water softener.

On my Diamante Plus, I fill it with salt about every 6 months and have it set to 5-7 grains. I've never had the salt light come on. But I do get the Rinse Aid light every 2-3 months, as I have it set to only dispense 1ml.

You should pour in the entire box when filling the salt dispenser. Pour in a full box the next time the light comes on (ours doesn't call for more salt very often--maybe twice a year at most?). It will seem like it will overflow, and comes close, but it won't.

FYI, run the DW (rinse and hold or short cycle) after filling with salt. This will wash away any errant salt grains.