Upon initial connection between a Gateway as SFTP client and a ProFTPD server, the transfer seems to freeze.

ProFTPD sets the channelwindowsize to 4GB at default(!)
This value is too big for a 32Bit Integer (0xff ff ff ff + 1). If the SFTP-Client only reads 32Bits out of the reply it reads 0. The proftp-Forums lists several non working SFTP cients and points to the possible solution:

The line

SFTPClientMatch .*XFB.* channelWindowSize 10MB

at the right position in the configuration of proftpd (usually /etc/proftpd.conf) reduces the channel window size for a client with "XFB" in the identification string.