Thanks for your report.
I have sent you an email with a development version of WinSCP to address you have used to register on this forum.

Although I get the warnings periodically, the transfers do complete eventually, so perhaps this is not a big problem.

Hi, I recently had this problem and it was sorted out, thanks again Martin!

HOWEVER, I have just upgraded my computer to Windows 7 64-bit (from 32-bit) and the problem has recurred.

I tried using 5.6.1, which was working for me before, and also tried the development build which was supplied (winscp20150401rtimeout), but the problem still occurs.

I took a log from 5.7.2, 29MB compressed - if you give me permission again I will upload it (the old password was denied).

WinSCP versions: 5.7.2, 5.6.1, 20150401rtimeout
started: moving from Windows 7 32-bit to 64-bit
windows version: Windows 7 64-bit
transfer protocol: SFTP
GUI: commander style
error: Host is not communicating for more than 15 seconds
steps: just start a file copy, it usually does a few tens of K and then gets stuck