RHEL7: "leapp upgrade" fails in reboot phase with message "BLS file /boot/loader/entries/XXX.conf already exists" then system reboots automatically

Issue

• After rebooting once leapp upgrade executed successfully, the system reboots after printing the following messages on the console:

  upgrade[XXX]: ============================================================
  upgrade[XXX]:                            ERRORS
  upgrade[XXX]: ============================================================
  upgrade[XXX]: <DATE AND TIME> [ERROR] Actor: zipl_convert_to_blscfg
  upgrade[XXX]: Message: zipl-switch-to-blscfg execution failed with non zero exit code.
  upgrade[XXX]: Summary:
  upgrade[XXX]:     Details: Command ['systemd-nspawn', '--register=no', '--quiet', '-D', u'/var/lib/leapp/el8userspace', '--bind=/etc/hosts:/etc/hosts', '--setenv=LEAPP_NO_RHSM=0', '--setenv=LEAPP_EXPERIMENTAL=0', '--setenv=LEAPP_COMMON_TOOLS=:/etc/leapp/repos.d/system_upgrade/el7toel8/tools', '--setenv=LEAPP_COMMON_FILES=:/etc/leapp/repos.d/system_upgrade/el7toel8/files', '--setenv=LEAPP_ENABLE_REPOS=rhel-8-for-s390x-baseos-rpms,rhel-8-for-s390x-appstream-rpms', '--setenv=LEAPP_UNSUPPORTED=0', '--setenv=LEAPP_EXECUTION_ID=...', '--setenv=LEAPP_HOSTNAME=...', '/usr/sbin/zipl-switch-to-blscfg'] failed with exit code 1.
  upgrade[XXX]:     Stderr: Host and machine ids are equal (<MACHINE_ID>): refusing to link journals
  upgrade[XXX]:             BLS file /boot/loader/entries/<MACHINE_ID>-XXX.conf already exists
  upgrade[XXX]:     Stdout:
  upgrade[XXX]: ============================================================
  upgrade[XXX]:                        END OF ERRORS
  upgrade[XXX]: ============================================================