BuildError(): error building a user of type

The problem seems to correlate to enabling FileVault for the whole disk. I did that for my MBP since I use it for work and in the case it gets lost, I need the content to be protected.

When you boot from a disk completely encrypted by FileVault, the OS cannot boot until the password to decrypt the disk content is entered.

To do this, OS X starts a minimal LoginManager where you enter your user name and user password. The entered user password is used to decrypt the FileVault Key and FileVault can now decrypt the contents of the disk and the real boot sequence starts.

Usually this LoginManager offers a flag to select the language for its GUI and the Keyboard Layout. As it seems, Apple made an error by not providing the needed resources on the unencrypted partition for setting up the LoginManger. This results (in my humble opinion) in the following error messages at boot time (only to see for a very short time):

**** ERROR _CreateMenuWithWithIdentifier no image for file: 1_flag_menu_item.png
**** ERROR _CreateMenuWithWithIdentifier no image for file: 92_flag_other_menu_item.png
**** ERROR UIFlagPickerRestoreState No state found for flagpicker
**** ERROR AchiveViewCreateWithOptions ArchiveCopyPNGImage failed for file:  preferences_good_samaritan_message_ribbon.png
**** ERROR AchiveViewCreateWithOptions ArchiveCopyPNGImage failed for file:  loginui_bootprogressbar.png

When you logoff a user and get back again to the LoginManager AFTER the system has booted, you will see the flag, the language menu, and the power off menu, which you don't see directly after switching on the computer.

It's a consequence of protecting with Firevault your boot disk. if it is enabled the EFI is not allowed to read or write from the boot disk at that early stage in the EFI initialization Sequence.

My best guess at this moment is that this behavior is intended, a new security feature present in OS X 10.11 El Capitan.

Apple will need to publish an EFI update to get ride of the deprecated "warning messages".