During this week the weak plant almost died. Photos I will add at the end of the week. I didn't know what is going on so I put her into quarantine and under grow lamp for 24 hours with UV and it almost died. Learning by doing.... The second plan is doing well.
@Simian5,
No idea, if the plant is older, I would suspect, overdosing on the nutrients but on such a young plant, I really don't know. The good message is, that it is not spreading and the new leaves are free of it.