What solved this problem for me (not saying is going to to it for everybody else) was to delete everything in C:\Users\"your user name"\AppData\Local\Temp folder. There were 3 or 4 files starting with dropbox..... (can't remember the exact names) which I couldn't delete only after restarting windows in Safe Mode. So, I deleted everything in Temp folder while in Safe Mode, rebooted normally and voila, dropbox started after logon on it's own. Cheers!
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