Dropbox is sending out the below email to all Dropbox users with new MacOS Sierra. The mail is received by those who are using Dropbox folder in MacOS Documents.

You have 2 options to avoid any dataloss.

Disable iCloud Sync of Documents Folder
Move your Dropbox folder from Documents to home directory.

Sample email below

Hi Admin,

We’re reaching out because we’ve detected one or more symlinks in your Dropbox. These symlinks could lead to data loss if no action is taken, as the recent release of macOS Sierra has changed the way Dropbox is able to sync files in some circumstances.

To make sure all your files are safe, please follow these steps:

Make sure you’re not using a symlink in Dropbox to a location also synced by iCloud, like Desktop or Documents. Alternatively, you could turn off iCloud sync.
Make sure your Dropbox folder is not located in Desktop or Documents. To prevent any issues, move your Dropbox folder to the default location.
Update to the latest version of the Dropbox desktop client (version 11.4.22). To ensure Dropbox continues to sync properly, download the latest versionof the desktop client.
Manage warning notifications. After you move your Dropbox folder to its default location, you may see warning notifications from iCloud when you move files from an iCloud-synced folder into Dropbox. You can opt out of these notifications by following these steps.

Why is this happening?

As part of Apple’s new Optimize Storage feature on macOS Sierra, iCloud creates “placeholder files” to free up disk space on Mac devices. These placeholder files, which have the extension .iCloud, take up very little disk space because they don’t have any of the information that makes up the actual file. You can think of a placeholder file as a map to the actual file—the placeholder file lives on your device, while the actual file it represents lives on the iCloud server.

When you have a symlink in Dropbox that points to an iCloud-synced folder like Desktop or Documents, it can cause serious issues. In this scenario, if iCloud replaces actual files with .iCloud placeholders, Dropbox will sync that placeholder instead of your actual file—so neither Dropbox nor iCloud will have the actual file.

To address this situation, please follow the steps listed above. For more information on Dropbox and iCloud on macOS Sierra, visit our help center.

Thanks,
The Dropbox Team