Hello again and thanks for your comments,
Elisei was right - some lines inside the group where on a different layer than the group itself.
After editing the lines towards the group layer, erasing worked as usual.
I don`remember to have a similar problem before.
Anyway, the layer status should have no influence, if a line is erasable or not.
thomthom , thanks for looking into it. I tried to reproduce this conflict but so far i wasn`t successful on this matter.
I will post the model, as soon as I have a similar case
Thanks again,
Uli