You are probably facing this bug:
https://winscp.net/tracker/1583
Please upgrade.

Using Version 5.9.4 of Winscp nuget package. Sometimes while calling PutFiles, it throws this error. It seems to succeed in writing the file, but I"m not sure what it's trying to do internally here. Does a new version of the package solve this?

Stack Trace:

System.Xml.XmlException: '.', hexadecimal value 0x00, is an invalid character. Line 18, position 28.
at System.Xml.XmlTextReaderImpl.Throw(Exception e)
at System.Xml.XmlTextReaderImpl.ParseText(Int32& startPos, Int32& endPos, Int32& outOrChars)
at System.Xml.XmlTextReaderImpl.ParseText()
at System.Xml.XmlTextReaderImpl.ParseElementContent()
at WinSCP.SessionLogReader.OpenLog()
at WinSCP.SessionLogReader.DoRead()
at WinSCP.SessionLogReader.Read(LogReadFlags flags)
at WinSCP.ElementLogReader.Read(LogReadFlags flags)
at WinSCP.SessionRemoteException.ReadMessages(CustomLogReader areader)
at WinSCP.SessionLogReader.Read(LogReadFlags flags)
at WinSCP.ElementLogReader.Read(LogReadFlags flags)
at WinSCP.SessionElementLogReader.Read(LogReadFlags flags)
at WinSCP.ElementLogReader.Read(LogReadFlags flags)
at WinSCP.Session.PutFiles(String localPath, String remotePath, Boolean remove, TransferOptions options)