I got this error while using the previous GUI version I had running. Today I am getting this using the latest GUI version downloaded today as well as CSharp code via the DLL.
Happens all the time in C# code via DLL.
Happens intermittently in your GUI.
FTP port 21
Passive and Active
I've attached WinSCP_Error_20140505_1517.zip which holds two log files and two screen caps. One log file created by Debug logging via your GUI and one created via CSharp app setting session logging.
Although I have masked private info in the log files, the attachment here is flagged as Private File.
Win7 Ultimate SP1, 2GB RAM
Visual Studio 2010