Hey there @Gigasoft! So this is a bit of a fun one, it’s recommended when you generate large maps to enable edit layers on generation due to how much overhead it can have if you already have materials and brushes attempting to change the height maps.
What’s happening is that while the map itself is changing over, it’s creating images to be the layers for each of the height maps/brushes it needs. So it turns your default map into the base, adds a layer for water, then adjusts the water layer. On a massive map that’s being rendered all at once this can be a bit of a struggle. Once the map is successfully over to edit layers, you should be able to save and change level, this should clear any held up Memory and Vram, then come back. You may need to reduce your draw distance as well depending, or even pause rendering in the viewport while you’re doing this to maximize your Vram usage while all of the operations are running.