In the case. this could be because of the htaccess file. This has happened during the handler change from suPHP to fcgi. To get rid of the error, modification is needed for the .htaccess file. Verification of the vbulletin site can be done via the index.php. When the index.php is openeed, at the top, you could find the vbulletin version.
Now backup the cureent .htaccess file as .htaccess.bak. Replace current .htaccess with the below one,
=--==--==-=-=-=-=-=-=-=-=-=-
RewriteEngine On
RewriteRule [^/]+-[\d]+/.+-([\d]+)/index([\d]+).html showthread.php?t=$1&page=$2 [NC,L]
RewriteRule [^/]+-[\d]+/.+-([\d]+)/ showthread.php?t=$1 [NC,L]
RewriteRule [^/]+-([\d]+)/index([\d]+).html forumdisplay.php?f=$1&page=$2 [NC,L]
RewriteRule [^/]+-([\d]+)/ forumdisplay.php?f=$1 [NC,L]
=--==-=--==-=-=-=-=-=-=-=-=-
Done
Now backup the cureent .htaccess file as .htaccess.bak. Replace current .htaccess with the below one,
=--==--==-=-=-=-=-=-=-=-=-=-
RewriteEngine On
RewriteRule [^/]+-[\d]+/.+-([\d]+)/index([\d]+).html showthread.php?t=$1&page=$2 [NC,L]
RewriteRule [^/]+-[\d]+/.+-([\d]+)/ showthread.php?t=$1 [NC,L]
RewriteRule [^/]+-([\d]+)/index([\d]+).html forumdisplay.php?f=$1&page=$2 [NC,L]
RewriteRule [^/]+-([\d]+)/ forumdisplay.php?f=$1 [NC,L]
=--==-=--==-=-=-=-=-=-=-=-=-
Done