Please post the entire exception stacktrace? And how exactly are you reading that file? Using java.io.File? Instead for JavaEE applications, it's better to use the following code:
String startupFileName=...// get the file name from ENC
InputStream is = this.getClass().getClassLoader().getResourceAsStream(startupFileName);
The resource will then be looked within the classpath of the application. So you can either package that myserver.properties within your app or place it in JBOSS_HOME/server/servername/conf folder (which by default is available in classpath).