TFB,
Where/when are you getting those errors? Is it as soon as you access the shop, at checkout, when you add to the cart, etc etc?
Secondly, what other mods/addons if any do you have installed?

Ed.
I don't use the Paypal Express checkout, but the discussion here:

http://www.zen-cart.com/forum/showthread.php?t=58104

and the text associated with the 'Auto select cheapest' seem to indicate that it could return you to the shipping page if it's confused? viz:

'When customer returns from PayPal, do we want to automatically select the Cheapest shipping method and skip the shipping page? (making it more *express*)Note: enabling this means the customer does *not* have easy access to select an alternate shipping method (without going back to the Checkout-Step-1 page)'

I'll do a bit of digging.

Chuck