HTTP ERROR 500 on final step of placing order (after Paypal)

Hello,
I’m having a lot of trouble to make this plugin work. I’m successfully redirected to the Paypal Express Checkout Page after adding products to the cart, and I can fill all the fields with no problems whatsoever. However, when clicking on the button to place the order (either “Pay Now” on Paypal or “Place Order” on woocommerce when going back to an Order Review page, I’m getting an error 500.
Checking the plugin log, nothing seems to be wrong, it just won’t proceed to the next step.
I already tried turning all the other plugins off, updating all I could, checking logs and nothing seems to tell me clearly what’s going on. Any ideas?
Thank you!