You many need to use the /C on the comspec and you may need to use a single % since you are calling it from a command line and not from a batch file. Does the command work from a command prompt if you call it EXACTLY as you have it here in the code? Verify with using a variable as the command without the SHELL, then calling the variable so as to show on screen what the command is trying to run.

I have found that trying to call a FOR command from a shell out from KiX is difficult trying to get all the proper quote marks, as they do make a difference to the FOR command as well.

If you have too much trouble you may want to try creating a batch file on the fly and shelling out and running the batch file.