Macbook Pro with bootcamp cannot boot to windows 10 after updating to Big Sur

I have a MacBook Pro (Retina, 15-inch, Mid 2014), with a Windows 10 bootcamp installation. It has been working fine for years.

Then I update the Mac OS to Big Sur 11.6.5, and then I couldn't boot to Windows any more, even if I hold down the alt/option key during startup - the boot options did show up (at first), but after I select the Windows Diamond icon, it just freezes. Sometimes, holding the alt key just won't bring up the boot option screen.

To be precise, at the beginning I tried and failed to boot to Windows. On the next day, it can boot straight to Windows. When I thought the problem was solved, after a few reboots I can't get to Windows any more.

I am able to set the startup disk as the bootcamp volume in "System Preferences", but it didn't help. I did grant the Full Disk Access right to System Preferences.

The "diskutil mount diskXsX" and "diskutil unmount diskXsX", plus holding alt and reboot several times thing, also didn't work.

I mount the EFI partition and in the EFI folder, I can only see APPLE, but not MICROSOFT, is this reflecting the exact root of the problem?

On the plus side, booting into Mac, I can still see the files of Windows in Finder.

Also, doing gdisk, gpt and fdisk (all through sudo) on /dev/disk0 won't work:

fdisk: /dev/rdisk0: Operation not permitted

GPT fdisk (gdisk) version 1.0.4

Problem opening /dev/disk0 for reading! Error is 1.

Output of diskutil list:

/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.3 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                 Apple_APFS Container disk1         81.4 GB    disk0s2
   3:       Microsoft Basic Data BOOTCAMP                418.7 GB   disk0s3

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +81.4 GB    disk1
                                 Physical Store disk0s2
   1:                APFS Volume Macintosh HD - Data ... 19.4 GB    disk1s1
   2:                APFS Volume Preboot                 312.5 MB   disk1s2
   3:                APFS Volume Recovery                623.5 MB   disk1s3
   4:                APFS Volume VM                      1.1 GB     disk1s4
   5:                APFS Volume Macintosh HD - Data ... 15.3 GB    disk1s5
   6:              APFS Snapshot com.apple.os.update-... 15.3 GB    disk1s5s1

/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *2.0 TB     disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:                  Apple_HFS bak bootcamp            214.0 GB   disk2s2
                    (free space)                         134.9 MB   -
   3:       Microsoft Basic Data BTCM_DRIVER             6.0 GB     disk2s3
   4:       Microsoft Basic Data NTFSVOL                 1.8 TB     disk2s4
                    (free space)                         134.3 MB   -

To provide more information, the bootcamp did broke down once by me trying to enlarge one of the partitions. Although it was fixed at the end, I noticed a few errors during the /fixboot and /rebuildbcd commands, don't know if these cause the problem (please refer to the last page):

Cannot boot to boot camp Windows after sh… - Apple Community

I need to use the Windows OS to work soon and hope that this problem can be solved asap. Many thanks!