After posting this "solution" i realized it doesn't work, so I kept searching for this problem, and saw someone changing the contactform.php file
i've tested and it works on version 1.7.5.1
change file contactform.php in -> modules/contactform folder
this:
$this->trans('Message from contact form', [], 'Emails.Subject').' [no_sync]',
to this:
$this->trans('Message from contact form', [], 'Emails.Subject'),
i don't know if you still have to change the file AdminCustomerThreadsController.php or not, but you can start with contactform.php and if it doesn't work, you can change the other one too.