This issue has been added to the tracker:
https://winscp.net/tracker/1683

Please, set Session.Timeout to a longer value to avoid the problem.

Okay, got both files.

You cannot use .xml extension for a session log. Please using anything else, like .log.

I set those two properties. Unfortunately it only created a debug log and not a session log. I have attached the debug log. Does that help?

Please set Session.DebugLogPath and Session.SessionLogPath and submit both logs.

Here is some more info:
It is throwing in Session.cs because _process is null.
Here is the call stack:
> WinSCPnet.dll!WinSCP.Session.CheckForTimeout(string additional) Line 1753 C#
WinSCPnet.dll!WinSCP.PatientFileStream.Wait(ref int interval) Line 51 C#
WinSCPnet.dll!WinSCP.PatientFileStream.Read(byte[] array, int offset, int count) Line 36 C#
System.Xml.dll!System.Xml.XmlTextReaderImpl.ReadData() Unknown
System.Xml.dll!System.Xml.XmlTextReaderImpl.ParseElementContent() Unknown
System.Xml.dll!System.Xml.XmlTextReaderImpl.Read() Unknown
WinSCPnet.dll!WinSCP.SessionLogReader.DoRead() Line 96 C#
WinSCPnet.dll!WinSCP.SessionLogReader.Read(WinSCP.LogReadFlags flags) Line 53 C#
WinSCPnet.dll!WinSCP.ElementLogReader.Read(WinSCP.LogReadFlags flags) Line 53 C#
WinSCPnet.dll!WinSCP.SessionElementLogReader.Read(WinSCP.LogReadFlags flags) Line 23 C#
WinSCPnet.dll!WinSCP.ElementLogReader.ReadToEnd(WinSCP.LogReadFlags flags) Line 73 C#
WinSCPnet.dll!WinSCP.ElementLogReader.Dispose() Line 32 C#
WinSCPnet.dll!WinSCP.SessionElementLogReader.Dispose() Line 17 C#
WinSCPnet.dll!WinSCP.Session.Cleanup() Line 1321 C#
WinSCPnet.dll!WinSCP.Session.Dispose() Line 185 C#
ReceiverCompressor.exe!ReceiverCompressor.MainWindow.downloadFiles() Line 284 C#
mscorlib.dll!System.Threading.ThreadHelper.ThreadStart_Context(object state) Unknown
mscorlib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx) Unknown
mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx) Unknown
mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state) Unknown
mscorlib.dll!System.Threading.ThreadHelper.ThreadStart() Unknown

I am able to get WinSCP to throw a NRE internally by doing a SynchronizeDirectories() Sftp from a remote directory containing 90k files to a local directory containing 30k files. I can assist by running a debug version or provide you with log files if you wish.