Hi @FKósa ,
- Downgrade your Bios back to version 0062 and remove your supervisor password.
- Upgrade Bios again to version 0076 and set your password back.
- My advice is to using recovery method to this updates. https://www.intel.com/content/www/us/en/support/articles/000005532/mini-pcs.html
This is not supposed to happen; it sounds like the password was corrupted.
Going from BIOS 62 to 76 is a pretty big jump. I might have suggested that this be done in a couple of steps (i.e. 62 -> 69 -> 76).
I agree with Leon; just to be sure, you should reinstall BIOS 76 using the Recovery Method.
Hope this helps,