Why is the Red Hat Ceph Storage version 5 upgrade to version 6 failing with `Permission Denied` errors?

Issue

• While upgrading Red Hat Ceph Storage version 5 to version 6, following the procedure in the Upgrade Guide , the upgrade fails with Permission Denied errors:

  # ceph log last cephadm
   mgr.node2.fnnqba (mgr.24205) 1089 : cephadm [ERR] Unable to write ceph.node1.installer:/var/lib/ceph/f622e716-cf20-11ed-a773-fa163ecec936/cephadm.0c72c9ea6423376a4137b1aa3075977575d454e7196befd31610c8fa5b51c85d: scp: /tmp/var/lib/ceph/f622e716-cf20-11ed-a773-fa163ecec936/cephadm.0c72c9ea6423376a4137b1aa3075977575d454e7196befd31610c8fa5b51c85d.new: Permission denied
  Traceback (most recent call last):
    File "/usr/share/ceph/mgr/cephadm/ssh.py", line 222, in _write_remote_file
      await asyncssh.scp(f.name, (conn, tmp_path))
    File "/lib/python3.9/site-packages/asyncssh/scp.py", line 1032, in scp
      await source.run(srcpath)
    File "/lib/python3.9/site-packages/asyncssh/scp.py", line 552, in run
      self.handle_error(exc)
    File "/lib/python3.9/site-packages/asyncssh/scp.py", line 387, in handle_error
      raise exc from None
    File "/lib/python3.9/site-packages/asyncssh/scp.py", line 550, in run
      await self._send_files(path, b'')
    File "/lib/python3.9/site-packages/asyncssh/scp.py", line 532, in _send_files
      self.handle_error(exc)
    File "/lib/python3.9/site-packages/asyncssh/scp.py", line 387, in handle_error
      raise exc from None
    File "/lib/python3.9/site-packages/asyncssh/scp.py", line 528, in _send_files
      await self._send_file(srcpath, dstpath, attrs)
    File "/lib/python3.9/site-packages/asyncssh/scp.py", line 456, in _send_file
      await self._make_cd_request(b'C', attrs, size, srcpath)
    File "/lib/python3.9/site-packages/asyncssh/scp.py", line 427, in _make_cd_request
      await self.make_request(action, args.encode('ascii'),
    File "/lib/python3.9/site-packages/asyncssh/scp.py", line 302, in make_request
      raise exc
  

• As the above output shows, the cephadm command cannot perform an scp to a temporary folder under /tmp/var/lib/ceph.

• Why is this occurring? How to solve this issue?