there is a problem when a "&" is present in a link.
Checking it in newsletter editor the link is correct (a phpbb forum link), but when you click it in the email you receive, in the browser address the "&" is changed in "&", causing the phpbb message "this argument doesn't exist etc.".
correct link: ...viewtopic.php?f=27&t=339
wrong link generated by wysija: ...viewtopic.php?f=27&t=339