This is an issue that needs to be investigated more. It spun out of JENKINS-41425.

Basically ...

1. create a folder "Foo"
2. create a job in that folder named "Bar"
3. open the job in classic Jenkins and press the "Open Blue Ocean" button

Works on Chrome, FF and Safari 9.x. Doesn't work on Safari 10.x.

The BO URL for this job will be blue/organizations/jenkins/Foo%2FBar. It sounds like Safari 10.x is URL decoding the path and converting it into blue/organizations/jenkins/Foo/Bar, causing the user to see a 404. This is not happening on the other browsers tested (including the earlier version of Safari).