Did you add kyttias's welcome pm code? If so, add your username to where it says YourUsernameHere in the code. The same goes with message title and content. Ex. below:
PHP Code:
if($mysidia->input->post("submit")){
$document->setTitle($this->lang->success_title);
$document->addLangvar($this->lang->success.$mysidia->input->post("username").$this->lang->success2);
$pm = new PrivateMessage(); // Send them a welcoming message
$pm->setsender('SapphirePhoenix');
$pm->setrecipient(htmlentities(addslashes(trim($mysidia->input->post("username")))));
$pm->setmessage("Welcome", "Test text.");
$pm->post();
return;
}
The first error has nothing to do with it. It's the same problem I have as well.