The Dropbox Desktop Client will present an error message "Can't Establish Secure Internet Connection" when it is unable to connect to the internet.
From the affected computer:
Manually visit dropbox.com in a web browser. If a block page is detected, check for group membership. Add dropbox.com to the group's whitelist.
If SSL decryption is enabled:
The Dropbox Desktop Client makes use of certificate pinning, and is therefore not compatible with SSL decryption.
Add dropbox.com to your Cipafilter's Super Whitelist to prevent SSL Decryption.