Hi madej -
A remote diagnosis is always ungrateful.
I suspect this spots are caused by fruit or oleic acid (orange juice, fats, etc.) or sulfur compounds. If the silver is then stored uncleaned for an extended period of time, then it looks like this. As long as the surface is not attacked, it is no problem. A good silver polishing paste and some "elbow grease" will bring back the old splendor. Do not scratch - just polish!
That´s what you need: