Got a new machine and put Ubuntu on. Installed Dropbox, which started syncing, eventually it stopped and just showed status Syncing
both in the daemon $ dropbox status as well as in the toolbar status icon. Nothing got synced, even syncing-icons on folders were
not shown. Running $ dropbox filestatus in the dropbox root directory yielded a list of directories that all showed status unwatched.
Problem: Dropbox has a default maximum number of files that can be 'watched'. This number is, at the time of writing 100,000, which
is incredibly low. You have to update this setting to a higher number. I followed this thread.
dropbox stop
echo fs.inotify.max_user_watches=10000000 | sudo tee -a /etc/sysctl.conf; sudo sysctl -
dropbox start
Essentially, you modify the file /etc/sysctl.conf to contain the setting fs.inotify.max_user_watches=10000000, i.e. the maximum
number of files that can be watched is 10,000,000 in my case.
Afterwards, Dropbox started running again.