Netx incorrectly declares XML documents containing the exclamation mark ('!') invalid. As far as I know, the exclamation mark is not an XML reserved character so it should not need to be escaped.
I have attached and example jnlp which demonstrates this issue. It is a copy of the example jnlp file: http://jnlp.sourceforge.net/netx/sample.html but with an exclamation mark.
Sample jnlp file with an exclamation mark
Confirmed here. The UK's Open University uses JNLP (from Elluminate) for online tutorial sessions, and I have to manually edit the JNLP file, only because it has a <title>Elluminate Live!</title> with an exclamation mark.