I found the problem in the pDb_Signup.class.PHP
in this code
$this->_mail(
$this->participant_values[$email_field],
$this->_proc_tags($this->receipt_subject),
Participants_Db::process_rich_text($this->_proc_tags($this->receipt_body))
);
I had to replace
Participants_Db::process_rich_text($this->_proc_tags($this->receipt_body))
by
$this->_proc_tags($this->receipt_body)
to make it work.
I understand that there is a reason for process_rich_text. I would like to know what went wrong to avoid missing on something else.
Thanks