Could not connect to ftp server Operation timed out connect

Hi I have some troubles posting file’s to a partner using BC 4.7.

I can connect to the partner’s ftp using port 21, I can change the directory but when I try to get directory information (LS) or putting a file (PUT) I get the following exception: [B2BCORE.0064.9001] Could not connect to ftp server: Operation timed out: connect

additional information:
transfermode is binary
transfertype is passive
the server is behind a firewall and the ports 20 and 21 are open

The interesting thing is that its sometimes working when I use the “ftp”-flowservice, but it’s never working when I use the FTPTransport OutboundProcess.

Any ideas?

Kind Regards, Sascha