Sadly I can't recall which book it was as I was just trying the open with feature when I read about in this thread so just tried to open a book. With other books I tried it to get the crash log it didn't crash so now I am not sure if that was the actual problem
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Anyway after trying both I prefer the original crengine renderer. I was wondering if it will be possible to add mupdf render as an option in android directly as I prefer calibre companion app for browsing my files so would like to have the option to open in crengine or mupdf option in that rather than from inside koreader.