The project page of a job on jenkins includes three possibilities to access published TestNG results:

1) left command list
2) middle info list
3) TestNG trend chart

The URL to access jenkins is http://<server>/jenkins in my case.

All links in 3) are working fine having the form http://<server>/jenkins/job/<project_name>/<build_number>/testngreports
On the other hand all links on the page that is returned after using 1) or 2) generates links of the form http://<server>/job/<project_name>/<build_number>/testngreport which don't exist and end in a 404 error. Notice that the '/jenkins' part is missing.