Wow, this is a stubborn issue! If you'd like to try to pinpoint where the error is occurring, please try placing these 3 lines after the first new line, and then try to log in using the facebook log-in button. If you get redirected to the home page with an error message instead of a blank page, then move the 3 lines to be after the next new line until we can pinpoint which line is failing. Meanwhile I will send a note to other developers to see if anyone else has better ideas about what could be going wrong.
Here are the three lines:
Feedback::error([PLACE A VARIABLE NAME FROM THE PRECEDING LINE HERE]); header("Location: tiki-index.php"); die();