This is the scenario for me on Windows 10. Basically you pause RTP, run a test render, then try to Unpause RTP, it will say the RTP finished instead of continuing to respond to any changes.
Here it is in detail :
Open Terragen to the default TGD. Let the Terrain preview finish. Click RTP . Turn on Surface colors, Atmos, Lighting. Let RTP run for 10 seconds or so.
Pause RTP. Run a test render at the default render settings. Let the real render run 10 seconds or so, doesn't need to complete.
Abort the test render.
Go to the preview window and unPause RTP. It will say "finished"
Now toggling the Surface colors icon, Atmo icon, Lighting icon, has no effect. Make a Change to the Base Colors in Shader group. Try clicking the Surface Color in RTP, it still does nothing.
At this point have to resort to the Pause, Unpause, Surface Color, No Surface Color trick to get the RTP running again.
It is possible to turn RTP off and start over, but with complicated terrains set to preview HD detail this can be quite a while to get back to the point of being able to run RTP again. So the pause unpause, etc. is faster to get things going to see the effect of a color change for example.