I had a similar problem. I ended up having to contact paypal support and create a support ticket. They told me that the test account I set up wasn’t configured properly. They had me create a new seller account (normal account, not pro) and then the CSR “upgraded” it to pro.
I then entered the new api username, pw, and signature in WooCommerce settings.
My cart worked fine after that.
Hope that helps.
If you are using Jetpack, configure wordpress.com stats, and make sure check the box that says “Hide the stats smiley face image.”
