If you have been using Dropbox on a filesystem other than ext4, you have been probably getting annoyed by the following warning, in the past 2 months:
Move Dropbox location. Dropbox will stop syncing in November.
According to Dropbox, this is intended, and their official response is:
Hi everyone, on Nov. 7, 2018, we’re ending support for Dropbox syncing to drives with certain uncommon file systems. The supported file systems are NTFS for Windows, HFS+ or APFS for Mac, and Ext4 for Linux.
We’ve updated our desktop requirements accordingly here.
A supported file system is required as Dropbox relies on extended attributes (X-attrs) to identify files in the Dropbox folder and keep them in sync. We will keep supporting only the most common file systems that support X-attrs, so we can ensure stability and a consistent experience.
Which is the corporate bullshit version of saying "sorry we run into some obscure problems with some filesystems, so we only have the capacity to support one for now", but that is a big marketing and sales no-no, so instead they sell it as a "consistent experience" and "stability", and some random bit about xattrs, which every filesystem supports. Here's a workaround.