This exception will be reported and content not transformed to XML when Silk Performer is unable to transform some AMF3 traffic where short names setting is enabled on the Flex server.  

Short names allows the server to use an alias instead of the full class name and Silk Performer expects the full class name to identify the name and location of the class within the jar files specified for transformation. 

The correct class may exist within the jars but Silk Performer cannot map to the class from the alias.  It is possible to diagnose this issue in two ways:

In the Silk Performer script the alias is displayed in the comment beside the untransformed data:

In the record log the full stack trace is displayed and this highlights both the full name of the class Silk Performer is expecting and again references the alias in the raw data: enables Silk Performer to map additional aliases to the full class name.