The easiest way to reproduce is to use the PosixBackupDriver with a backup_posix_path = /var/cinder/permdenied	(where cinder does not have write access to the directory) Then you get

 ERROR oslo_messaging.rpc.server Traceback (most recent call last):
 ERROR oslo_messaging.rpc.server   File "/opt/stack/cinder/cinder/backup/manager.py", line 491, in continue_backup
 ERROR oslo_messaging.rpc.server     updates = backup_service.backup(backup,
 ERROR oslo_messaging.rpc.server   File "/opt/stack/cinder/cinder/backup/chunkeddriver.py", line 546, in backup
 ERROR oslo_messaging.rpc.server     volume_size_bytes) = self._prepare_backup(backup)
 ERROR oslo_messaging.rpc.server   File "/opt/stack/cinder/cinder/backup/chunkeddriver.py", line 332, in _prepare_backup
 ERROR oslo_messaging.rpc.server     container = self._create_container(backup)
 ERROR oslo_messaging.rpc.server   File "/opt/stack/cinder/cinder/backup/chunkeddriver.py", line 235, in _create_container
 ERROR oslo_messaging.rpc.server     self.put_container(backup.container)
 ERROR oslo_messaging.rpc.server   File "/opt/stack/cinder/cinder/backup/drivers/posix.py", line 105, in put_container
 ERROR oslo_messaging.rpc.server     os.makedirs(path)
 ERROR oslo_messaging.rpc.server   File "/usr/lib/python3.8/os.py", line 213, in makedirs
 ERROR oslo_messaging.rpc.server     makedirs(head, exist_ok=exist_ok)
 ERROR oslo_messaging.rpc.server   File "/usr/lib/python3.8/os.py", line 213, in makedirs
 ERROR oslo_messaging.rpc.server     makedirs(head, exist_ok=exist_ok)
 ERROR oslo_messaging.rpc.server   File "/usr/lib/python3.8/os.py", line 213, in makedirs
 ERROR oslo_messaging.rpc.server     makedirs(head, exist_ok=exist_ok)
 ERROR oslo_messaging.rpc.server   File "/usr/lib/python3.8/os.py", line 223, in makedirs
 ERROR oslo_messaging.rpc.server     mkdir(name, mode)
 ERROR oslo_messaging.rpc.server PermissionError: [Errno 13] Permission denied: '/var/cinder/permdenied'
 ERROR oslo_messaging.rpc.server 
 ERROR oslo_messaging.rpc.server During handling of the above exception, another exception occurred:
 ERROR oslo_messaging.rpc.server 
 ERROR oslo_messaging.rpc.server Traceback (most recent call last):
 ERROR oslo_messaging.rpc.server   File "/usr/local/lib/python3.8/dist-packages/oslo_messaging/rpc/server.py", line 165, in _process_incoming
 ERROR oslo_messaging.rpc.server     res = self.dispatcher.dispatch(message)
 ERROR oslo_messaging.rpc.server   File "/usr/local/lib/python3.8/dist-packages/oslo_messaging/rpc/dispatcher.py", line 309, in dispatch
 ERROR oslo_messaging.rpc.server     return self._do_dispatch(endpoint, method, ctxt, args)
 ERROR oslo_messaging.rpc.server   File "/usr/local/lib/python3.8/dist-packages/oslo_messaging/rpc/dispatcher.py", line 229, in _do_dispatch
 ERROR oslo_messaging.rpc.server     result = func(ctxt, **new_args)
 ERROR oslo_messaging.rpc.server   File "/opt/stack/cinder/cinder/backup/manager.py", line 525, in continue_backup
 ERROR oslo_messaging.rpc.server     volume_utils.update_backup_error(backup, str(err))
 ERROR oslo_messaging.rpc.server   File "/usr/local/lib/python3.8/dist-packages/oslo_utils/excutils.py", line 227, in __exit__
 ERROR oslo_messaging.rpc.server     self.force_reraise()
 ERROR oslo_messaging.rpc.server   File "/usr/local/lib/python3.8/dist-packages/oslo_utils/excutils.py", line 200, in force_reraise
 ERROR oslo_messaging.rpc.server     raise self.value
 ERROR oslo_messaging.rpc.server   File "/opt/stack/cinder/cinder/backup/manager.py", line 497, in continue_backup
 ERROR oslo_messaging.rpc.server     self.message_api.create_from_request_context(
 ERROR oslo_messaging.rpc.server   File "/opt/stack/cinder/cinder/message/api.py", line 140, in create_from_request_context
 ERROR oslo_messaging.rpc.server     self.create(context=context,
 ERROR oslo_messaging.rpc.server   File "/opt/stack/cinder/cinder/message/api.py", line 104, in create
 ERROR oslo_messaging.rpc.server     action[0],
 ERROR oslo_messaging.rpc.server TypeError: 'NoneType' object is not subscriptable
 ERROR oslo_messaging.rpc.server