TL;DR - It looks like the new UI has a wild redirect when trying to add a user key.
Whilst using the new UI me and my team tried adding a user SSH key by clicking the button “Authorize with GitHub”. After clicking this, it would lead you to the CircleCI homepage.
At first we thought it was a config issue on our end, but when we tried to do this later on in the old UI (by clicking “Add user key”) it would allow us to do so and we successfully connected the user and the CircleCI builds.
There was indeed an issue in the new UI that was preventing users from adding SSH keys. As you found out, the workaround was to temporarily switch back to the legacy/old UI.
I’m happy to confirm that we have recently implemented a fix for this issue, and you can now add your SSH keys via the new CircleCI UI.