Hi Oshyan,
Troubleshooting results:
First off, for #2, the only way to get the file to open was to close the preview window while the DEMs were loading, or cancel the DEMs loading themselves. After that, playing with the file and opening the preview window went fine and yielded no errors, until that file was saved and re-opened. Then, again, it did not open.
So, trying #1, I put each of the heightfield and heightfield loaders into TG clips and loaded them individually into a fresh file. The end result is that they can both be inserted into the file, they can both be turned on, they can each themself be plugged into the Compute Terrain node, but when one heightfield is plugged into the second heightfield (in either order), and then into the Compute Terrain, while the result will properly show and work in the preview window, upon saving, closing, and re-opening the file, it will crash.
So somehow, having both heightfields plugged into each other is causing the issue in TG4.1.11. Occasionally, TG will simply give the preview window message "Aborting threads" and hang. At other failed attempts, TG gave the error message "An unknown error occurred in trBucketRender::RenderMore() rasterized pass floatn3" without completing the preview window (it remained gray).
Does that help figure out what it is?
Thanks,
Matt
ADDITIONALLY: The two DEMs are 450 MB each, but this made no impact on TG4.0.0.4, and I had been using the same file in TG3.