When the integrated Windows Firewall is active, upload works as expected. However downloads of large files (tested with file sizes more than 1 GB) always fail after 99% of transmission. The connection is terminated and the FTP server denies access. After a few seconds and an error message the download process restarts again. This is an endless loop.
Workaround: Deactivate the Windows Firewall while downloading. Maybe this is a server dependend problem using Strato server backup.serverkompetenz.de.