A few days ago I updated Dropbox app from Goolge Play. Now when I try to sign in the updated app, I never succeed. It will always prompt that "Sign in failed, can't connect to dropbox. Please make sure you are online and try again " , but actully I can sign in dropbox.com with firefox on this phone without any problem, so I think this is not about the network.
I also tried clear the cache and data, or reinstall Dropbox app, no luck.
I find there was someone who had the same problem, and it didn't seem to be solved.