We found that having two-factor authentication enabled (paypal security key or similar token) causes login problem. Here's another site reporting the same problem:
We reached out to paypal to find another workaround because the only currently working workaround is disabling security key.
Update: Workaround by one of our users:
I logged in to PayPal on my Web browser seperately and then signed in through riftcat and it worked.
Please try the free version before buying!