BER, I think I figured it out
I didn't understand why I had BER only on the potted ones. Although I had some good yeilds from it, to many had BER. Then I noticed a new Italian Plum plant which I planted late (end of June) started producing fruit just last week.
I noticed there is BER on them too.
I came to the conclusion that it is the soil contributing to it. I used Miracle Grow Potting mix which might have been to strong for the 4 week old plant.