I have been having issues with linux dropbox for a couple of weeks. My server ran a copy of crunch bang ++ for the last year or so to back up files. My desktop and an old netbook had the same. Then desktop and server went into "syncing" mode and stuck. As dropbox said this was a none supported os both have been rebuilt several times with Ubuntu LTS. Checks on a clean copy work until a reboot or the preferences are changed to a larger folder in which case "syncing" is all you get. Netbook is then the only working copy.
Just to rule out the possibility, can you check if you have any symlinks within your Dropbox folder with the steps outlined in point 7 here?
If you don't have any within the local Dropbox folder, can you check if your local folder contains more than 300,000 files? You can check out a more info on why this can affect the desktop app syncing from here.
Since this occurs when trying to add more folders to sync to your desktop through the selective sync settings, can you try adding a few subfolders at a time (instead of the whole folder if it's large) to see if that helps?