Why convert2rhel fails with 'SystemExit: Error(21): Is a directory Diagnosis: N/A'?

Issue

• convert2rhel fails with error.

  [2024-01-18T12:48:26+0000] TASK - [Prepare: Backup package files] 
  #************************************
  Backing up /etc/sysconfig/authconfig.
  Backing up /etc/logrotate.d/yum.
  CRITICAL - Error(21): Is a directory
  ERROR - (ERROR) BACKUP_PACKAGE_FILES::UNEXPECTED_ERROR 
  - Unhandled exception caught
  Description: Unhandled exception was caught: Error(21): Is a directory
  Please file a bug at https://issues.redhat.com/ to have this fixed or a 
  specific error message added.
  Traceback: Traceback (most recent call last):
  File "/usr/lib/python2.7/site- 
  packages/convert2rhel/actions/__init__.py", line 572, in run
    action.run()
  File "/usr/lib/python2.7/site- 
  packages/convert2rhel/actions/pre_ponr_changes/backup_system.py", 
  line 113, in run
    backup.backup_control.push(restorable_file)
  File "/usr/lib/python2.7/site-packages/convert2rhel/backup.py", line 
  173, in push
    restorable.enable()
  File "/usr/lib/python2.7/site-packages/convert2rhel/backup.py", line 
  370, in enable
    loggerinst.critical("Error(%s): %s" % (err.errno, err.strerror))
  File "/usr/lib/python2.7/site-packages/convert2rhel/logger.py", line 1 
  70, in _critical
    sys.exit(msg)
  SystemExit: Error(21): Is a directory Diagnosis: N/A
  Remediation: N/A