The error message was introduced in version 2.12.4 and appears in the Agent UI, but is NOT reported to the Management Console.
This error message means that the Agent has a main server configured (through custom parameter priority_peers in the Agent profile), but cannot establish or has lost connection to this server, or all main servers if several are configured. Also, the error is reported if the configured main server is not added to the job.
It is not the same as connection from the Agent to the Management Console!
What it means?
Since the priority_peers parameter is added to the Agent Profile, most likely this main server is the only one that has all the copies of files and other agents in the mesh are in Selective Sync mode and thus have only files' placeholders.
Such setup configuration does not guarantee that there's at least one copy of new or updated files in the mesh. It's highly not advisable to remove, rename or move any files and subfolders while you see this error.
What to do next?
Raise this message error with Resilio Administrator, to check connectivity from the Agent to the main server.
Also, check job configuration and make sure that the main server is added to the job at all.
Error code: NO_PRIORITY_PEERS_ONLINE