Question: Error in using Worksheet:-Comparator

Hi everybody, 

I am working with Maple 2015.2 ; system = OS X El Capitan
I would like to compare the outputs of two files by using  the Worksheet:-Comparator command

OriginalFile.mw file contains this single instruction
> a:=1;

ModifiedFile.mw contains this single instruction
> a := 2;

Both us are located on my desktop, which is also my current directory in the worksheet from which I am going to run the comparison.

MyDir := cat(currentdir(), "/Desktop/");
OriginalFile := cat(MyDir, "OriginalFile.mw"):
ModifiedFile := cat(MyDir, "ModifiedFile.mw"):

# to verify that all is going well I use these two commands 
# Worksheet:-DisplayFile(OriginalFile):
# Worksheet:-DisplayFile(ModifiedFile):

Running the  command  Worksheet:-Comparator(OriginalFile, ModifiedFile) returns me error messages :
The first one is  (after having selected  "compare" and next "run" in the Comparator Maplet) 

and the second one (after having cliked ont the "OK" button" on this alert window)

Note that the same thing happens if I use Worksheet:-Comparator() and select the two files with the browsers
Could you please help me to understand what happens ?

Thanks in advance