Restart DSM first (make sure you saved any design changes prior). This should reload the DirectX plug-in.
If the issue persists, try going into to File->DesignSpark options-> Popular and lower rendering setting to 4 or 5.
In DSM 4.0 and newer:
Then, go into Advanced-> Renderer and switch to OpenGL from DirectX. Restart DSM again after changing this.
If you have a computer with switchable graphics, assign a different graphic card to be used with DSM (switch from one which is causing the grey box to appear).