If set to a non-empty string nanomsg will print errors to stderr. Some errors will be resolved by nanomsg itself (e.g. if nanomsg can’t establish connection it will retry again in a moment). Some depend on the environment (e.g. port that is bound by another process need to be released). In any case nanomsg will not repeat the error message until error is clear and appear again (e.g. connection established then broken again).