SFTP upload not enabled for your plan
The site allows SFTP, but this plan does not. Auth, tier-gate, and import worker can all return this when the member’s ul2_ftp_on_* switch is off.
Why this happens
Users & limits → FTP enabled is off for Registered / Premium / Premium Pro as applicable. Guests never get SFTP. Site mod is already on (otherwise you would see SFTP upload disabled on this site).
Diagnose and fix
1. Admin → Settings → Users & limits — enable FTP/SFTP for that tier (toggle.ul2_ftp_on_reg / _prem / _prem_pro).
2. Per-user override if only one account should differ.
3. Or have them upgrade to a tier that has FTP on.
4. After save, restore/provision credentials if needed and ask them to reconnect.