I understand you want to test-signed driver with a digital signature during development but failed with error about BCD, right?
Try to rebuild BCD after boot into recovery, and then perform the following command:
• c: (assuming this is where the Boot folder is)
• cd boot
• attrib bcd -s -h -r
• ren c:\boot\bcd bcd.old
• bootrec /RebuildBcd
I noticed you said "I have reinstalled Windows 10 from an old backup.", do you mean you created a clone image for Windows 10 system?
As clone will bring potential system risk and clone image is also out of our support scope, we recommend to setup system with a valid Windows 10 disk or installation media. Or we could capture system image with DISM command line tool after generalized image with Sysprep.
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