I bought a brand new MacBook and set it up earlier today.
Went through setup, updated to High Sierra, enabled FileVault with a recovery key, wrote that down and then shut it down at some point. When I tried to log in again, it wouldn't let me. Wrong password. OK, I thought, I have the recovery key. So I log in using that, change my password, restart and run into the same issue again. New password doesn't work. Next time I tried with a fool-proof password "123456" - no luck. The password somehow gets reset to something I don't know on restarts.
Next I tried to reinstall my system but I can't do that because the HD is locked with the password I don't know. Then I figured I'd try to disable/reenable FileVault but that also doesn't work as suddenly my account, despite being the only account and admin, isn't eligible to change FileVault settings.
What can I do here?