If your account is located on Windows platform, the current error means that there is some misconfiguration with the SECURE FOLDERS installation in your Plesk panel. Therefore, please access your Plesk panel and re-install your secure folders from the beginning, including recreation of secure users. If this solution didn't help, please contact our support team by opening a trouble ticket or sending an e-mail. Likely some permission problem has occurred and it's possible to solve it from the administration area only.