When using Jenkins with a custom Home directory (ex: /home/jenkins/jenkins-prometheus), it appears that a 502 Bad Gateway error occurs when clicking on the Class Name on a testng test result. Here is an example of the URL that the plugin is trying to navigate to:

[domain]/jenkins/job/OAM%20-%20Positive%20Testing/8/testngreports/automationFramework/Example_NewUserRegistration

Changing the URL to the following successfully loads the page for the Class Name:
[domain]/jenkins-prometheus/job/OAM%20-%20Positive%20Testing/8/testngreports/automationFramework/Example_NewUserRegistration

It appears that the problem is related to the custom Home directory.