Problem with PayPal checkout *urgent*

So, I got myself MyCafeCommerce the other week, replacing my old Zen Cart store. One of the reasons I decided to go with MCC was that they had PayPal as one of the checkout options, setup and pre-defined. Good.

But after having gone through weeks of painstaking setup and adding of products, tweaking of settings etc, I notice that I can't go through with any payments. I get an error message:

"This transaction is invalid. Please return to the recipient's website to complete your transaction using their regular checkout flow."

Why is this?

[Before you go down that path: yes, I AM aware that it's not the usual password etc, the API is setup correctly]

I have chosen the option "PayPal Standard" in the CafeCommerce credit card settings. There are also the options "PayPal Advanced (US only)" and "PayPal Website Payments Pro Hosted (UK/AU only), but since I live in Sweden those don't seem to be options for me. My PayPal account is PayPal Business, and I didn't think that would be a problem, is it?

I would be thankful for a quick solution to this so I can set up my store properly and welcome my waiting customers.

Thanks,
Daniel