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

Hi Martin,
I posted two log files(one use proxy and another use none proxy),
---------------------------------
File Protocal:FTP
Encryption:SSL Explicit encryption
Proxy Type:SOCKET5
WinSCP Version:WinSCP 5.2.2
OS Version:Windows Server 2008 R2
---------------------------------

with proxy via SSL,the .txt file can be uploaded successfully ,but the file size is ZERO(local .txt file size is very small);

with proxy via SSL,the .txt file can be uploaded successfully , the file size > ZERO(local .txt file size is big);

with none proxy via SSL, everything is ok.

I have compared the two log file,I found that with proxy the log file exist a log-line :"Error retrieving server address, cannot test if address is routable",but another log file not exist.

TEST:
Use proxy
1.when the .txt file size is 4K, uploaded to FTP server size be showed ZERO;
2.when the .txt file size is 49K,uploaded to FTP server size > ZERO;

Thanks a lot!

Regards,
Frank