Couple of questions :
Assuming this is inbound flow, where the message is coming to sFTP channel from a external partner, '
- is the error while picking the message ? and do you see 2 entries in the wire message report ?
- is the error while posting to the backend application ?.. The error you posted looks like the issue during the message post to the back end application, as it has reference AQ, IP_IN_Queue.
Suggestion to use JMS Q as the integration between B2B and backend application. This can be enabled in administration -> configuration -> Use JMS Queue as default (True)