Landscape Section greyed out

I am working with UE4 2.24.3 I have created a landscape (Resolution: 2017 x 2017): I created a landscape material, assigned it to the Landscape and created the matching Target Layers (8 Layers). I am using two layers for Splines. I face the following problem: with some layers which I only use manually (not as the initial layer nor for the splines), when I want to paint on the landscape, the blending keeps compiling and finally, some segments stay greyed out as in the screen shot. It seems to me that the layer cannot be blended at all in this particular segment. It is all fine in other segments. When I undo the paint step, then all is fine (the prevous blending comes back). I have cleared the entire layer and made a do-over for this particular layer, but the segments which were greyed out before would grey out again when I apply this layer to that segment. It is quite frustrating.

Any idea what could be wrong? Any idea how to fix it that I can apply all layers to all segments of the landscape?

Thank you for your help!