This is something I had noticed in my scalability test but at that time I thought I may have done something stupid and later, I saw a similar note in support case.
I have two portal servers (ports-01, ports-02) configuration where nullPortletPage.war is predeployed before servers are started. If I start both servers simultaneously and access http://localhost:8280/portal/portal/default/NullPortletPage then I get a 404 error with "The requested resource () is not available". I am able to access the page if I hit http://localhost:8180/portal/portal/default/NullPortletPage (notice the port) .
What is more interesting is that when I start servers one by one and access portal page as server comes online i.e. start second server when first one has already started and page has been accessed, then I can access the NullPortletPage in both servers. Since there is a work around, let's fix this by 2.6.5 release.