Maybe qualify the location of the php file then in Styles & Tempalates>Style Manager>YOUR STYLE>Post Thank You Templates>post_thanks_button at the top you will see <a href="post_thanks.php......etc maybe qualify it like this <a href="http://www.YOURSITE.COM/FORUM/post_thanks.php....etc
Just a shot in the dark, without beig able to see that all the files have been ftp'd to the correct places, checking your styles....etc i don't think i can help further, sorry