When using SyncBack with FTP, and receive this error, it is because there are too many connections to your FTP server. The maximum number of connections is configured on the FTP server itself. There are three possible solutions:
Configure the FTP server to increase the maximum number of connections.
Wait for the other connections to close.
If the FTP server is yours, and you have cPanel, then login to your cPanel and go to FTP Session Control to disconnect the FTP connections.
SyncBack only uses one connection per profile except with SyncBack V8 or newer. It can be configured to use multiple simultaneous connections when scanning the FTP server (see the setting Scan threads on the FTP->Advanced settings page). By default it will only use one connection.