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

A)

each user should be able to disable it by disabling DST: Settings, general settings, date & time options; Select DST Correction always off

B)

Or globally as admin, Go to AdminCP, Styles & Templates, Search in Templates, search for text: dst_correction, open up template "footer" and replace "dst_correction" in it by something else: dst_correction00000
(it should not trigger that redirect to empty page profile.php?do=dst and should not modify the date)
Maybe would be needed to AdminCP/Maintenance/Clear system cache, or clear cache on Cloudflare if used or Ctrl+F5 reload page)

C)

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]" />

D)

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

E)

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

F)

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

G)

try to define global timezone for all users to one that does not use DST? I assume adding to some header or footer template:
date_default_timezone_set("Europe/Zagreb");