Summary: const newJobUrl = `${baseUrl}/view/All/newJob` in PIpelines.jsx is hard coded and doesn't take into account localised URIs in jenkins.
This should not be hard coded. This link should actually be removed from blue ocean header in longer term as it gets its own creation screen and doesn't have to delegate to classic.

Fresh install of jenkins on my osx laptop.
Installing plugin via experimental repository, everyting's ok.

• Jenkins : http://localhost:7070/
• Blue Ocean : http://localhost:7070/blue/ then automatic redirect to http://localhost:7070/blue/pipelines
• Click on "new pipeline" : http://localhost:7070/view/All/newJob --> 404

Opened an issue https://groups.google.com/forum/#!topic/jenkinsci-dev/C7Ze59o_XOo and guys redirected me to you.