Tried current and it doesn't crash! Yay!
It's a memory exhaustion problem. Boo.
Code:
Rendered /tmp/calibre_3.48.0_tmp_gttcnB/UUtd8B_comic_extract/v01/048.png
Failed /tmp/calibre_3.48.0_tmp_gttcnB/UUtd8B_comic_extract/v01/049.png
Failed /tmp/calibre_3.48.0_tmp_gttcnB/UUtd8B_comic_extract/v01/050.png
Failed /tmp/calibre_3.48.0_tmp_gttcnB/UUtd8B_comic_extract/v01/051.png
Failed /tmp/calibre_3.48.0_tmp_gttcnB/UUtd8B_comic_extract/v01/052.png
Failed /tmp/calibre_3.48.0_tmp_gttcnB/UUtd8B_comic_extract/v01/053.png
Failed /tmp/calibre_3.48.0_tmp_gttcnB/UUtd8B_comic_extract/v01/054.png
Failed /tmp/calibre_3.48.0_tmp_gttcnB/UUtd8B_comic_extract/v01/055.png
Failed /tmp/calibre_3.48.0_tmp_gttcnB/UUtd8B_comic_extract/v01/056.png
Failed /tmp/calibre_3.48.0_tmp_gttcnB/UUtd8B_comic_extract/v01/057.png
Failed /tmp/calibre_3.48.0_tmp_gttcnB/UUtd8B_comic_extract/v01/058.png
Rendered /tmp/calibre_3.48.0_tmp_gttcnB/UUtd8B_comic_extract/v01/059.jpg
Rendered /tmp/calibre_3.48.0_tmp_gttcnB/UUtd8B_comic_extract/v01/060.jpg
Rendered /tmp/calibre_3.48.0_tmp_gttcnB/UUtd8B_comic_extract/v01/061.jpg
Rendered /tmp/calibre_3.48.0_tmp_gttcnB/UUtd8B_comic_extract/v01/062.jpg
QImage: out of memory, returning null image
Failed /tmp/calibre_3.48.0_tmp_gttcnB/UUtd8B_comic_extract/v01/063.png
So looks like my options are to rewrite the rendering process to use RAM more efficiently and don't do that. I'll stick with "don't do that" for now.
Thanks.