If there is a problem accessing the Jenkins web UI, of course it is going to be difficult to diagnose those by going to that same UI and getting a bundle. As an alternative, you can use the Jenkins CLI to obtain a bundle from a shell. Before you encounter problems, make sure you have downloaded jenkins-cli.jar from the server (go to /cli/ to get a link). You may also need to make sure you have authenticated to the CLI, for example by uploading an SSH public key to your Jenkins user account. Then try:

java -jar /path/to/jenkins-cli.jar -s http://server/ support

This will generate a support bundle, save it to your local computer, and print the filename. You can also request only specific components to include; to see the currently available list, ask for the command’s help:

java -jar /path/to/jenkins-cli.jar -s http://server/ help support