So i have been working for a time in this project and on this terrain and sudenly when i painted it a bigg part of the teraid stoped to load the textures/shaders and i have tryed to reload them but still the gray boxes.
Are your samplers set to shared wrap? It is possible you are still going over the limit due to other samplers (for instance lightmaps), but it may not show the error in the preview window since the layer weights will not show up on the preview mesh, and the landscape material compiles out unused layers. So it is likely showing you only a single compiled layer of the material in that preview. Often you can have too many layers and the problem only shows up once you actually paint all the layers onto the same section of terrain. If you complete erase a layer from a section, the layer is compiled out.