Paypal Gateway - Sandbox won't work in live environment?

Strange problem - I’ve just developed a site in Simplecart. The Paypal gateway works fine in my localhost development environment in both Sandbox and Live modes, but when I migrate the site to production environment, the Live mode works fine but the Sandbox doesn’t. When I click Paypal, I just get a message “No token passed”…

My localhost environment is XAMPP on Windows.
This always appears in the Modx error log regardless of whether it’s Sandbox or Live mode

[2016-10-25 06:06:36] (ERROR @ /core/xpdo/om/xpdoobject.class.php : 811) simpleCartOrderProduct: Attempt to set NOT NULL field productcode to NULL
[2016-10-25 06:06:36] (ERROR @ /core/xpdo/om/xpdoobject.class.php : 811) simpleCartOrderProduct: Attempt to set NOT NULL field productcode to NULL
[2016-10-25 06:06:36] (ERROR @ /core/xpdo/om/xpdoobject.class.php : 811) simpleCartOrderField: Attempt to set NOT NULL field value to NULL
[2016-10-25 06:06:36] (ERROR @ /core/xpdo/om/xpdoobject.class.php : 811) simpleCartOrderField: Attempt to set NOT NULL field value to NULL
[2016-10-25 06:06:36] (ERROR @ /core/xpdo/om/xpdoobject.class.php : 811) simpleCartOrderField: Attempt to set NOT NULL field value to NULL
[2016-10-25 06:06:36] (ERROR @ /core/xpdo/om/xpdoobject.class.php : 811) simpleCartOrderField: Attempt to set NOT NULL field value to NULL

The installations are identical so it doesn’t seem like it’s a coding problem.

Any help would be appreciated?

Thanks in advance
Rob