Trust your nose and your tongue. Smell it, taste it; if it smells and tastes fine, then it's probably fine. (Strawberries are rather acidic and jam often includes lemon/lime juice, so the odds that it's something harmful are rather small.)

That said if I had to take a guess: you didn't skim off the foam while you were cooking the jam, and that foam jelled itself. If that's correct it's safe to eat.