As you probably know, Dropbox has ended support for folder syncing to drives with "uncommon" filesystems, which on Linux means anything but Ext4. As a result, Linux Dropbox users can't synchronize files if the Dropbox folder is on Btrfs, Ext3, ZFS, XFS etc., and even eCryptfs on top of an Ext4 filesystem. You can see the Dropbox desktop system requirements here.
In case you can't switch to a different cloud storage provider or use an Ext4 filesystem without eCryptfs for your Dropbox folder location, you can use a simple tool, called dropbox-filesystem-fix, that works around this Dropbox limitation, allowing the Dropbox desktop client to continue synchronizing files, no matter the filesystem type in use.