Having JAI both in the webapp and in the JRE/JDK prevents startup with Jetty 9.
A simple workaround is to remove JAI from GeoServer WEB-INF/lib as suggested in the install guide (the difference being that right now the step is no more optional):
So, two options:
Update the docs
See if changing the classloading options in Jetty makes it start anyways
Mass closing all resolved issues not modified in the last 4 weeks