[xdebug-general] PHP page not fully loading when using xdebug

Hi all. I am a new xdebug user, and I'm having some trouble getting a
trace from a PHP application (vBulletin). So long as xdebug is not
enabled, the site works fine. However, as soon as I enable xdebug (By
adding the following line to .htaccess: php_flag xdebug.auto_trace On)
the PHP page fails to load. I do get a trace, but it ends after 460
lines and it is obvious that it is still very deep in the call stack
when the script terminates:
$ tail /tmp/trace.2043925204.xt
    0.0515 7268664 -> trim()
~/forum/includes/class_core.php:2544
    0.0515 7268736 -> htmlspecialchars()
~/forum/includes/class_core.php:2544
    0.0516 7268848 -> preg_replace()
~/forum/includes/class_core.php:2544
    0.0516 7267408 -> define() ~/forum/includes/init.php:290
    0.0516 7267432 -> defined() ~/forum/includes/init.php:294
    0.0522 7321192 ->
require_once(~/forum/includes/class_hook.php)
~/forum/includes/init.php:303
    0.0522 7320312 -> defined() ~/forum/includes/init.php:304
    0.0522 7320200 -> vBulletinHook::set_pluginlist()
~/forum/includes/init.php:311
    0.0523 7320864 -> vBulletinHook::fetch_hook()
~/forum/includes/init.php:331
    0.0523 7320960 ->
vBulletinHook::fetch_hook_object() ~/forum/includes/class_hook.php:107

With 15 spaces before -> on the last line, that looks like the stack
is 6 layers deep. I do not have read access to the system log nor to
the httpd log, and for some reason I cannot redirect logging with
php.ini directives in .htaccess (I am formatting them correctly).

What else can I check? Why should the application run differently when
xdebug is enabled?

Thanks.

-- 
Dotan Cohen
http://gibberish.co.il
http://what-is-what.com