When sending trackbacks:

Warning: Cannot modify header information - headers already sent in /home/***/public_html/blog/inc/_core/_template.funcs.php on line 174
Warning: Cannot modify header information - headers already sent in /home/***/public_html/blog/inc/_core/_template.funcs.php on line 182
For the trackbacks to be sent, an admin should load the site (backoffice or frontend), then the trackbacks will be processed.

Note:
As of this writing, I'm still using the public release 3.2-beta.