I was having problem when trying to enable FileVault, and found out later that because of Nix. In order to enable it again, in terminal:
- List all uses from fdesetup with this command: sudo fdesetup list
- Remove user that starts with prefix nixbld from fdesetup: sudo fdesetup remove -user <nixbld_user>
- Enable FileVault: sudo fdesetup enable
If success, you should get prompt to enter your username (with admin permission) and password. You can check encryption status with: sudo fdesetup status, and it should show you the current progress for turning on FileVault.
Not sure if need to add back nixbld user to fdesetup, but probably will find out later if there is problem.