Some time ago Dropbox began to warn me about supporting ext4 as FS only. The base file system needs to be ext4, and specifically not ecryptfs. i.e. if your home folder is encrypted, you need to put the Dropbox folder someplace else, eg. a separate ext4 partition.
To overcome this, you can use an utility called dropbox-filesystem-fix. This makes your Dropbox folder appear as if it's on an unencrypted Ext4 filesystem, no matter what filesystem you use, and you don't have to mount anything, you just have to run Dropbox with with a dropbox-filesystem library (LD_PRELOAD).
You'll need to grab the code from GitHub, compile the library (make) and replace the Dropbox startup entry with the dropbox_start.py script provided by dropbox-filesystem-fix.
If you need complete step by step instructions, check out this page from Linux Uprising blog.