I have tested this problem a little more out:
In the first minute I´m painting on the landscape, the video memory increases to about 6gb, after this point, it increases only slow, after 3-5minutes the editor crashes with the message above…
If I save the changes before it crashes, the complete video memory gets emptied.
Maybe there´s no problem with the grasstool, but with the video memory management?
Edit: It happens with a new Landscape too!