overcloud configuration update fails with "the field 'args' has an invalid value ({'monitor_keyring': [...]"

Issue

• We modified number of overcloud templates
• We ran openstack overcloud deploy --templates ....and all the rest of the templates
• Ansible fails after a certain time with

        "TASK [ceph-mon : generate monitor initial keyring] *****************************",
        "Monday 22 February 2021  12:48:51 +0100 (0:00:01.424)       0:13:29.596 ******* ",
        "TASK [ceph-mon : get initial keyring when it already exists] *******************",
        "Monday 22 February 2021  12:48:51 +0100 (0:00:00.059)       0:13:29.655 ******* ",
        "fatal: [overcloud-controller-0]: FAILED! => {\"msg\": \"the field 'args' has an invalid value ({'monitor_keyring': \\\"{{ (initial_mon_key.stdout | from_json)[0]['key'] if initial_mon_key is not skipped else monitor_keyring.stdout }}\\\"}), and could not be converted to an dict.The error was: Expecting value: line 1 column 1 (char 0)\\n\\nThe error appears to be in '/usr/share/ceph-ansible/roles/ceph-mon/tasks/deploy_monitors.yml': line 26, column 3, but may\\nbe elsewhere in the file depending on the exact syntax problem.\\n\\nThe offending line appears to be:\\n\\n\\n- name: get initial keyring when it already exists\\n  ^ here\\n\"}",