Java Web Start 6 is unable to interpret proxy.pac file correctly when app is run on JRE 1.4.2

Java Web Start 6 Update 18 is used to launch a Java Web Start application,
that requires to be run on JRE 1.4.2:

    <j2se version="1.4" java-vm-args="-client"/>

An automatic proxy file (proxy.pac) is not interpreted correctly, when the
application is started via jnlp file:

      javaws.exe app.jnlp
or
      javaws.exe /path/to/app.jnlp

When using Java Web Start 6 with an URL however, the correct proxy
is picked and everything works as expected.

     javaws.exe http://server:port/app.jnlp
or
     javaws.exe file:\/\/c:\/app.jnlp