Your default simultaneous transfer limit is on 3. Leave it and Upload or Download multiple files at once. Sometimes files are uploaded or downloaded with 0.00 KB (not all of them). I need to re upload them to fix it. I can personally fix it permanently when i change the simultaneous connections on just 1. This error based on SFTP Connection. Not using FTP-Connections. So cant say if the error exist there too.
Version: 3.4.6