Error 2016 is caused when your login request is being blocked by either antivirus or restricted networks. The most common antivirus that causes this error is Kaspersky Antivirus. The ‘Safe Money’ feature can cause error 2016 if enabled. You can test this by disabling Safe Money using the steps on Kaspersky's site here.
Restricted networks such as University or Work could be blocking a specific IP address we use when you log in. If you are on a University network, contact the Campus ISP/IT Department and provide the following IP ranges to see if they can whitelist their connection to our servers.