I am always fascinated with how different applications report the same problem.
I find it tremendously valuable to ‘break’ applications or cause intentional errors to see how applications behave what they report, or not report.
The behavior and reported error will vary from product to product and from version to version, so it helps to repeat your test every so often. Don’t forget that some applications can use multiple protocols and may mask, or misreport error messages.
The same point applies to your tool of choice. Does your protocol analyzer report on any ftp or application error codes? And lastly are there any other protocol clues to help zero in on this issue, like a TCP RST, ICMP error message, etc..
In this video, I use FTP and various FTP clients as an example of this methodology.