No job jar file set. User classes may not be found. See JobConf(Class) or JobConf#setJar(String)

 No job jar file set.  User classes may not be found. See JobConf(Class) or JobConf#setJar(String)

This happened when the class was set (which was the popular fix suggested).

In my case extracting the external jars into the jar when exporting the jar helped.