There is a closed thread on this in archieves, but unresolved prior to forum upgrade.
I'm having the same problem, this is following installation of the updated timezone offset. Note: this is a fresh install, no upgrades.
error is generated only on checkout:
Fatal error: Call to a member function on a non-object in /../../../../includes/classes/order.php on line 854
The original files (including order.php) were replaced as per timezone offset instructions. the configuration table appears ok.
Only way to not get this error is to go back to using the original order.php, thus not able to employ the correct time zone.