dropbox cannot establish secure connection

According to Dropbox's help desk:
Your system time is off
Your computer will compare your system date to the expiration date of our SSL certificate. If your system time is stuck in the past or so far in the future that it is outside of Dropbox's SSL certificate expiration date, a secure connection will not be established. This is easily corrected by changing your system time to the current date.


You have a proxy or firewall blocking the Dropbox service

Dropbox uses standard internet ports (80 and 443) to transfer data. However, many firewalls and security software will proactively block unauthorized or unknown internet services. Add Dropbox to your proxy or firewall settings as an exception to connect to the Dropbox service.

http://forums.techguy.org/networking/1078394-dropbox-cannot-establish-secure-connection.html