All connections to the Dropbox service are done using a secure connection (SSL). If you get an error that says "Cannot establish a secure connection," it is probably due to one of the following reasons:
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 (such as antivirus programs) 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.
Known issues with ESET Endpoint Security Solutions: According to ESET's website, this product currently has a conflict with Dropbox when SSL scanning is enabled. Possible workarounds are to exclude the Dropbox certificate within the "Ask about non-visited sites" mode, as well as to exclude dropbox.exe from protocol filtering as shown here.
Known issues with Kaspersky Endpoint Security 8: This product currently has a conflict with Dropbox that prevent users from connecting after the application is updated. The current workaround is to add an exclusion in Kaspersky to allow Dropbox to connect. The second option is to upgrade the application to the Kaspersky Endpoint Security 10 version. More information on Kaspersky's site.
Known issues with HitmanPro (anti-malware): This product currently has a conflict with Dropbox that can block Dropbox sync operations. There are no known workarounds for this, however you can temporarily disable HitmanPro when you'd like to sync your Dropbox. Visit the HitmanPro website to learn more about the product, or to report false positives.
Your default proxy settings are incorrect
Some external software programs will change your computer's default proxy settings when installed and may not alert you of this. Confirm that your proxy settings haven't changed and if you don't use a proxy setting, select the option for "No proxy".