CONTAP-229682: [Unable to open "server.cfg"] error occurs during ONTAP rebooting of FAS2720

Issue

[Unable to open "server.cfg"] error occurs during ONTAP rebooting of FAS2720. The details are as below:
1.Downgrade ONTAP from 9.13.1P3 to 9.8P21 by selecting [(7) Install new software first] from the boot menu.
2.Select [(4) Clean configuration and initialize all disks] from the boot menu.
3.boot_ontap via loader:
[LOADER-A> boot_ontap
Loading X86_64/freebsd/image2/kernel:0x200000/1142320 0x317000/11852552 0x1065000/3935112 0x1425b88/13423904 0x200240/1016 Entry at 0xffffffff80317000
Loading X86_64/freebsd/image2/platform.ko:0x20f4000/4784992 0x2584360/611808
Starting program at 0xffffffff80317000
NetApp Data ONTAP 9.8P21
ena_rss_init_default_deferred() [TID:100000]: No devclass ena
IPMI device unit 0 rev. 1, firmware rev. 10.00, version 2.0, device support mask 0xbf
IPMI device unit 1 rev. 1, firmware rev. 10.00, version 2.0, device support mask 0xbf
Copyright (C) 1992-2023 NetApp.
All rights reserved.
*******************************

• *
• Press Ctrl-C for Boot Menu. *
• *
  *******************************
  cryptomod_fips: Executing Crypto FIPS Self Tests.
  ....Snipped....
  No firmware was updated, so no need to reboot
  SSAL: tss_tpm_nvread:977
  Unable to open "server.cfg" <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
  FAIL 　　　　　　　　　　 <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
  Apr 18 12:00:57 Power outage protection flash de-staging: 42 cycles
  Pensando Offload Driver, ver 1.4.0-E-78
  Pensando Ethernet NIC Driver, ver: 1.4.0-E-78
  ionic_rdma ver 1.4.0-E-78 : Pensando RoCE HCA driver
  ....Snipped....]