Attach, bitte, your file!
How could he attach the file if he is not able to save it correctly?
Does it happen with every file you try to save? Does it also happen if you open an already existing, saved file and you try to save it again?
Which file name do you try to save the file to? Did you try using a simple filename like "abc" (you should not need to type the extension *.mcdx).
Anyway, I have no idea what the reason what the reason for your problem may be, but I suggest that you contact PTC support.
I can save the file if i have not written anything in it. But if i write anything in the file i cant save is. This error occurs.