La Caixa Sermepa - items ordered still in cart

We've installed the "Pasarela Sermepa - Caixa, CajaMadrid" add-on and everything seems to work except that when you've completed your order, all of your items are still in your cart. This isn't happening with other payment methods. Any ideas?

Additionally, we're having these problems with it:

I make the payment with the credit card, then properly redirected back to the site and informed about the order's success. In that moment the cart is empty and it keeps this way as long as I don't log out. If I log out and then log in, purchased items are again in the cart as if I've closed the session without making an order.

Also, I've found another problem. I've tried to make several incorrect credit card authentifications (cc number, vat code...) and I'm not properly redirected with any of them, the navigator gets stuck and I'm not sent back to the site to make another selection in payment.

Any idea? Thanks!