The workaround is to transfer files manually, which can be tedious if there are a lot of files.
The FTP connection aborts even though it is possible to perform a direct FTP connection from one server to another.
First, check if it is possible to FTP to another server.
If you can, try rebooting the servers – this was taken as a troubleshooting step. After the restart, the job was tested and ALL of the files were transferred, and the error was no longer present.