If TFTP gives an error message as
Path cannot be null. Parameter name: path Illegal characters in path.
It means that you are giving the root directory in the argument. TFTP has a root directory and everything that is being fed to it should be relative to it.
If your root is
And your request is
C:\tftp_root\my\folder then you should change it to
In case you need to go out of your scope, like
C:\Users\foo, then you should do