This error message was introduced in version 2.12 and may appear in Agent UI when user quits the Agent which has a priority_peers setting configured for it. Most often it's used in a setup with Transparent Selective Sync shares.
Basically, this error message means that the Agent hasn't yet uploaded the files to the main server (one of the priority_peers) or hasn't yet merged the folder tree with it.
For example, a new file was added to the folder on this Agent. Other Agents in the mesh have Transparent Selective Sync enabled. Until the file is uploaded to the main servers, other agents won't have any place to download it from if this Agent quit.
It's safe to quit the Agent anyway, bearing in mind the following:
- if there are other agents awaiting to download the new file, they won't have any source of file available.
- other Agents' status may stay on 'syncing' as they will be waiting for files' availability.
The Agents will resume syncing once this Agent appears back online and finishes upload.