The job's artifact package was being delivered to an agent reporting to one execution server from a retrieval agent that reports to a different execution server. Based on this finding we looked and confirmed that the two execution servers weren't configured to communicate with each other (which is necessary to get a file/artifact across).
The error is often associated with NES Servers not being configured to properly communicate with each other. However, this error has also been seen (as seen in NES logs appearing with a message: Could not create proxy file sender for.*with id [.*_es_<remote_Nes_Hostname>] already exist) while running on versions earlier than 6.6.0.10228. When this message is present, it is not a configuration problem.
There are a few solutions to this kind of problem:
If you're getting this error(s) then be sure you're using version 6.6.0.10228 or greater. If not, apply cumulative fix 10228 or greater if you're on Nolio version 6.6. Or, apply the latest cumulative fix if you're on 6.7. Then, if the error(s) continues and you're sure it is not a configuration issue (especially if you're getting the "sender for.*with id.*already exist" message) then please open an issue with Technical Support.