Public summary:

Stack failure due the this error:

2020-05-04 18:17:35,866 p=2797 u=mistral |  TASK [tripleo-ceph-fetch-dir : ensure we create a new fetch_directory or use the old fetch_directory] ***
2020-05-04 18:17:35,866 p=2797 u=mistral |  task path: /usr/share/ansible/roles/tripleo-ceph-fetch-dir/tasks/create.yml:70
2020-05-04 18:17:35,866 p=2797 u=mistral |  Monday 04 May 2020  18:17:35 +0000 (0:00:04.500)       0:30:56.160 ************ 
2020-05-04 18:17:35,920 p=2797 u=mistral |  fatal: [undercloud]: FAILED! => {"changed": false, "msg": "Received HTTP: 407 when attempting to GET from http://<redacted>:8080/v1/AUTH_<redacted>/overcloud_ceph_ansible_fetch_dir/temporary_dir.tar.gz?temp_url_sig=<redacted>&temp_url_expires=<redacted>"}

The code where the the curl command is failing on;
  /usr/share/ansible/roles/tripleo-ceph-fetch-dir/tasks/create.yml

~~~
 63 # swift backup
 64 - when: local_ceph_ansible_fetch_directory_backup | length == 0
 65   block:
 66     - name: attempt download of fetch directory tarball from swift backup
 67       shell: "curl -s -o /tmp/{{ old_ceph_ansible_tarball_name }} -w '%{http_code}' -X GET \"{{  swift_get_url  }}\""      <<-----HERE
 68       register: curl_get_http_status
 69       ignore_errors: true
 70     - name: ensure we create a new fetch_directory or use the old fetch_directory
 71       fail:
 72         msg: "Received HTTP: {{ curl_get_http_status.stdout }} when attempting to GET from {{ swift_get_url }}"
 73       when:
 74         - curl_get_http_status is changed
 75         - curl_get_http_status.stdout != "200"      # deployment update
 76         - curl_get_http_status.stdout != "404"      # new deployment
 77     - name: unpack downloaded ceph-ansible fetch tarball to fetch directory
 78       # unarchive module hit https://github.com/ansible/ansible/issues/35645
 79       shell: "/usr/bin/gtar --gzip --extract --file /tmp/{{ old_ceph_ansible_tarball_name }} -C {{ playbook_dir }}/ceph-ansible/fetch_dir"
 80       when:
 81         - curl_get_http_status is changed
 82         - curl_get_http_status.stdout == "200"
 83     - name: remove downloaded ceph-ansible fetch directory tarball from filesystem
 84       file:
 85         path: "/tmp/{{ old_ceph_ansible_tarball_name }}"
 86         state: absent
 87       when:
 88         - curl_get_http_status is changed
 89         - curl_get_http_status.stdout == "200"
~~~

This is fixed by patches tracked in duplicate bug 1769719