CiviMail - when Track Click Throughs is enabled, links result in a white screen with warnings
Richard's answer is correct.  While this seems unrelated, look at this answer for an explanation of what's happening here, except instead of a PDF header, it's redirect code.  You're getting an unrelated warning, and you can't send a warning and then send a redirect header.  In fact, I'm guessing that you can't currently generate PDFs right now either.  Fixing the "unrelated" warning will fix this problem, and the problem is that your webhost hasn't defined a timezone for PHP.

Note that Richard's answer is only one way to set a PHP setting, and doesn't work if your webhost disallows it.  I would take up with your webhost the best way to resolve this.  Another way that works in almost any environment is to use an ini_set command anywhere in your civicrm.settings.php file.