When recording a Flex application, NeoLoad deserializes the binary stream between the browser and the server.
It will convert the binary data to an XML content and do the reverse operation so we are sure that NeoLoad can correctly handle it. If a request or response was not converted, you will have in your scenario some broken requests that will appear with a specific yellow icon.
After the recording, you have to follow the Post recording Wizard. This wizard will show you the names of the jar or classes which need to be uploaded to the Controller.
Once this is complete, you will not receive this error and all your requests and responses will be decoded to the xml format.
For more information, see "Adobe Flex/AMF requests" under "Design" section in NeoLoad documentation.