re. Broken Pipe issues without DRM plugin:
I don't know if it is relevant, but in developing a plugin on OS X I've hit this Broken Pipe error. In my case the plugin (or a library it used) was being too verbose and this caused some overflow and hence the exception.
Look at the calibre job's logs or your console logs to see if anything is being too verbose - if it is then try making it more terse (or ask the plugin developer perhaps) and see if that helps.
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