Windows Backup BitLocker Error 0x8031004A

I had Windows 10 with BitLocker drive encryption of both my main and backup drives.

I downgraded to Windows 7 and am using VeraCrypt on both of those drives now. Both drives are unlocked at boot.

I cannot seem to get Windows Backup to work. It gives me this error soon after starting any backup:

Check your backup
BitLocker Drive Encryption cannot be used because critical BitLocker system
files are missing or corrupted. Use Windows Startup Repair to restore these
files to your computer.

Error code: 0x8031004A

Startup repair does nothing. I wonder if some BitLocker signature is still on the drives somewhere, causing it to try and load BitLocker DLLs, which this version of Windows (Pro) does not have.

Anything else I can try? I'd like to just use Windows Backup instead of 3rd party backup.

Try completely cleaning your backup disk.

It's possible the issue is with the backup drive and not the drive that you are backing up. The instructions below will remove everything from the disk, ensuring no traces of Bitlocker encryption remain on the disk.

WARNING: These steps will completely erase the backup disk!

1. From an elevated Command Prompt run diskpart
2. Type list disk to show all connected disks.
3. Type select disk # where # is the disk number of your backup drive.
4. Type clean to completely erase the disk.

5. Run the following diskpart commands to create and format a new partition:

   create partition primary
   select partition 1
   active
   format fs=ntfs quick label="Backup drive"
   assign
   

6. Type exit to quit Diskpart.