I'm having this same issue on the 2.3 also. AWS support says: "target instances are likely to close TCP connections prematurely (with TCP RST or FIN) while the ALB has outstanding requests, causing 502 errors. This could be due to mismatched keep-alive timeout settings between the ALB and targets." The ALB is set to a 1m timeout so it could be NiFi closing it early. Looking at that next.  ... View more