You seem to be trying to use our FTP server with a web browser...
If your browser doesn't support FTP, that won't work.
Ideally, you should not use FTP. Please use sftp instead (also not expected to work in the browser, but the browser may launch WinSCP).
If you really want to use ftp, you should get a client that does, such as:
or you typed something wrong... try again: ftp://ftp.brockmann-consult.de:21/