This is a problem with your PHP configuration. ArrayObject is part of the standard library. Your PHP installation must be using --disable-spl.
I recommend you upgrade PHP on your server to 5.3.x (latest is currently 5.3.15). This upgrade is recommended anyways. And 5.3 no longer allows you to disable the standard library which should fix this error.
So basically this error shouldn't be possible on your current setup. Either your forum is running a different instance of PHP than the one you are checking, or this is some strange problem with your PHP configuration in which case I recommend reinstalling PHP on your server (contact your host or server person). ArrayObject is part of the standard library so it's a PHP problem and not a XenForo problem.