Adding <async-supported>true</async-supported>
to Push Servlet
solved this issue. Does not explain why it worked on why of my glassfish instances tho.
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.1" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaeehttp://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd">
...
<servlet>
<servlet-name>Push Servlet</servlet-name>
<servlet-class>org.primefaces.push.PushServlet</servlet-class>
<init-param>
<param-name>org.atmosphere.cpr.broadcasterCacheClass</param-name>
<param-value>org.atmosphere.cache.UUIDBroadcasterCache</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
<async-supported>true</async-supported>
</servlet>
<servlet-mapping>
<servlet-name>Push Servlet</servlet-name>
<url-pattern>/primepush/*</url-pattern>
</servlet-mapping>
...
</web-app>