The TestNG Plugin for Jenkins adds a new menu entrry to all projects called "TestNG Results" which, when clicked, leads to a page called "TestNG Results Trends" with the URL "/job/projectname/testngreports/".
On that page a HTML image map shows the trend and offers links to the single builds. However, the anchors in the imagemap are all like "BUILDNUMBER/testngreports/". What they should point to instead is "../BUILDNUMBER/testngreports" - note the "../".
I guess the devs simply reused code from the project overview widget, where the URLs work together nicely.
As it is now, however, the testng report page contains links to non-existent pages, leading to 404.