Dropbox can show you this message when it can’t update (and the icon in the system tray gets a red cross 
):
can’t establish a secure connection
In my case, it was because I was using the Fiddler2 tool to debug some web-traphic.
Fiddler intercepts the WinInet API layer by putting itself as a proxy in it.
The great thing is that Fiddler2 can hook HTTPS traffic.
The not so cool thing is that I forgot that DropBox routes its’ HTTPS traphic through the WinInet API too :-)
Disabling Fiddler2 enabled DropBox to start connecting 
again and finally making a good connection 
.
As soon as DropBox has a good connection, you can restart Fiddler2 again to debug your internet traffic.
–jeroen
via: Dropbox – Why cant I establish a secure connection? –.
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