Cant recover from Time Machine Disk because [UPDATE NEEDED]
from the answer to a very similar question from 2014:


  
  Log in into the Mac as administrator.
  Backup the Mac with Time Machine (optional but recommended)
  Launch System Preferences selecting System Preferences from the Apple menu.
  Click the Security &amp; Privacy preference pane.
  Click the FileVault tab.
  Unlock the pane clicking on the padlock (administrator password required)
  Disable FileVault and wait (…about 3hrs with a 250GB no SSD HD)
  Restart the Mac (optional but recommended)
  Go back the FileVault tab.
  Enable FileVault and wait (…about 7hrs for me!!!)
  Restart the Mac.
  The unknown user account will disappear.
  This should also now allow you to set automatic login.
  


I assume, if thats the only account listed, it has to be an administrator account.

I know you might not be able or want to back up your drive again, though. So maybe this isnt what youre looking for (though that step is optional).

also heres a discussion on apple.com from 2011 with a similar question and answer. this one may be closer to your scenario as it also doesnt mention any accounts besides the "update needed" account