I have re-uploaded my images again.. that fixed the problem temporarly.
Unfortunetely I did it already 3 x times in last 3 x days because the previews were missing again and again.
Re-uploading helps only temporarly.
Be aware of it that if you change the price, or someone comments on any of your models, the bug could be back and occur again on that item.
CGT doesn´t communicate on that bug anymore as it seems, haven´t hear from them back anymore.