I found this issue in ruffus 2.6.3 https://github.com/bunbun/ruffus/issues/65 also discussed here https://github.com/bunbun/ruffus/pull/67 ruffus 2.6.3 RethrownJobError don't follow the normal conventions and so its exception causes problems when they cross process boundaries. This change carefully examines the various forms of ruffus exception objects that can appear in 2.6.3 and parses them more carefully. It also removes any direct posting of the exception to the logger because this triggers another serializing of the exception object, mutating it further.