Is Your vBulletin 4 forum being redirected to a blank page profile.php?do=dst ?
People advise various things to fix this issue:

A)

Open up your footer template and find:

<form action="profile.php?do=dst" method="post" name="dstform">

..and under it add:

<input type="hidden" name="securitytoken" value="$bbuserinfo[securitytoken]" />

B)

add $vboptions['bburl'] before the /profile.php?do=dst
in the footer.

C)

add {vb:raw vboptions.bburl} before the /profile.php?do=dst
in the footer.

D)

add a 301 redirect to your .htaccess file or make a new file with the line;

Code:

Redirect 301 /profile.php http://www.yourdomain.com/forums/profile.php