Laptop forgets Windows fingerprint, then Windows password, then BIOS password

A relative asked me to look into his laptop because he could not log in anymore. The laptop model is Aspire A515-45G. It started with the login via fingerprint on Windows not working reliably. As a workaround, the password was used. Until login with the password started to make problems too, and stopped completely to work. Now logging into Windows is not possible anymore. I decided to start a live Linux distribution to backup the data and look around. For that, I had to disable secure boot. On this Acer laptop, it is only possible after having set a supervisor password in the BIOS, so I did. I

Question by ocroquette on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.