I found that my VIP two-factor authentication is incompatible with Express Checkout for at least some merchants. My payment will hang forever at the "logging in" screen. This problem goes away when I deactivate the security key on my account. The SMS authentication works fine.

This is something we should keep an eye on and test for.

Here's a useless forum on the topic:
https://www.paypal-community.com/t5/Access-and-security/PayPal-App-amp-Security-Key-log-in-issues/td-p/890587