XF 1.3 Can not access Admin cp (/admin.php) after upgrading to Xenforo 1.3.1

Hi all,
I upgraded my forum from 1.2.4 to 1.3.1, but after upgrading I could not access admin cp (/admin.php), it always inform: "The requested forum could not be found".
During upgrading, 1 warning displayed:
The following warnings were detected when verifying that your server can run XenForo:

Your server is running an outdated and unsupported version of PHP (5.2.17). If possible, you should upgrade to PHP 5.4 or 5.5.
Your server has disabled fundamental core PHP functions via the disable_functions directive in php.ini. This will cause unexpected problems in XenForo. All PHP functions should be enabled.

These may affect the proper execution of XenForo at times and should be resolved if possible.

I ignored and continued, this is the root cause for this issue ?
How can I fix it ?
(After upgrading, I asked hosting company to move my website to server using PHP 5.4 but nothing changed)

Thanks so much,