I have just tested with 5.4.5 and I get the same error.
I should also mention that I am using the one-page checkout, if you'd like to try to reproduce the issue
Upgrading to bleeding-edge / 1.4 is not ideal, as the production environment does have a theme already developed and some overrides (though still no third party modules). But I will attempt the upgrade to fix the paypal module, I need the guest checkouts working.
UPDATE: I have tested bleeding edge and the error happens there too.
To recap: this happens on a clean install of tb, bleeding edge, no overrides, many default modules disabled, niara theme, one page checkout. It also happens on 1.3.0 and with PayPal module version 5.4.5.
UPDATE2: provided screenshot of modules enabled
I believe the module does something to the cart because the cart is gone, but it is not successfully transformed into an order.