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I have a film image plane running in the background for reference but rendering as tif for transparency. Despite creating new cameras, changing the background, going through every setting I can think of, when I render in 2018.6, the background is solid red. My only remedy to this was to open the scene and render in 2019.
Take a look at the arnold log and you might see a warning or error message for why this failed. See
https://docs.arnoldrenderer.com/display/A5AFMUG/Log for help on getting log files.