This is the problem that I have. I tried this but it doesn't work for me. I have no idea what to do next. I can't access the setting either.
dropbox-dist/dropbox-lnx.x86_64-16.4.30/dropbox is running.
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If you right-click the sound indicator on the panel and select 'Remove' it will remove the 'Indicator Plugin'.
On reboot dropbox seems to detect that the plugin is missing and use another mechanism which works.
You do lose your sound indicator, but if you don't need sound or have volume-keys on your keyboard it is a good tradeoff
Perhaps someone knows another plugin that can be used to adjust speaker volume?