The request was aborted: Could not create SSL/TLS secure channel.

Based on the error message, your machine is having trouble establishing a secure connection to Dropbox. The Dropbox servers are being served with valid SSL/TLS configurations though, and I'm not aware of any current service disruptions that should be causing this. You may want to check if you have any firewall, proxy, VPN, anti-virus, or other security software, etc. that may be interfering with your connections.

Regardless, I see you're attempting to load the contents of the www.dropbox.com/oauth2/authorize page programmatically though. That is a web page that should be loaded directly in the user's system web browser. You should not retrieve it via code. Please refer to the OAuth Guide for more information.