I have this same problem, here is the callstack:
mscorlib.dll!System.IO.__Error.WinIOError(int errorCode, string maybeFullPath) + 0x321 bytes
mscorlib.dll!System.IO.FileStream.Init(string path, System.IO.FileMode mode, System.IO.FileAccess access, int rights, bool useRights, System.IO.FileShare share, int bufferSize, System.IO.FileOptions options, Microsoft.Win32.Win32Native.SECURITY_ATTRIBUTES secAttrs, string msgPath, bool bFromProxy, bool useLongPath) + 0x477 bytes
mscorlib.dll!System.IO.FileStream.FileStream(string path, System.IO.FileMode mode, System.IO.FileAccess access, System.IO.FileShare share) + 0x54 bytes
mscorlib.dll!System.IO.File.Open(string path, System.IO.FileMode mode, System.IO.FileAccess access, System.IO.FileShare share) + 0x25 bytes
WinSCPnet.dll!WinSCP.SessionLogReader.OpenLog() + 0xd1 bytes
WinSCPnet.dll!WinSCP.SessionLogReader.DoRead() + 0x64 bytes
WinSCPnet.dll!WinSCP.SessionLogReader.Read(WinSCP.LogReadFlags flags) + 0x86 bytes
WinSCPnet.dll!WinSCP.CustomLogReader.TryWaitForNonEmptyElement(string localName, WinSCP.LogReadFlags flags) + 0x5f bytes
WinSCPnet.dll!WinSCP.CustomLogReader.WaitForNonEmptyElement(string localName, WinSCP.LogReadFlags flags) + 0x29 bytes
WinSCPnet.dll!WinSCP.CustomLogReader.WaitForNonEmptyElementAndCreateLogReader(string localName, WinSCP.LogReadFlags flags) + 0x29 bytes
WinSCPnet.dll!WinSCP.Session.Open(WinSCP.SessionOptions sessionOptions) + 0x775 bytes