Could not load file or assembly 'file:///...\WinSCPnet.dll' or one of its dependencies. The system cannot find the file specified.

You may get this error when using WinSCP .NET assembly from restricted environments, such as SSIS. To use the assembly from SSIS, it needs to be installed to GAC.

If the above is not your case, use Assembly Binding Log Viewer (Fuslogvw.exe) to debug assembly loading.