Are you using the SFTP step that has been posted in communities (ssh-utils)?
If this is the custom step, is the JAR file in hotdeploy on every machine that uses this step?
I have also seen this type of error when one or more definition in the .lisaextensions file (packaged with the source code) does not match correctly with the Java class names.
And, since this is DT 7.5, the custom extension should be compiled using the Java 7 compiler.