I guess the data in file diff.txt is not in a proper format. I'm not that acknowledged in this spehre of math, you know. But Mathematica keeps giving me the same error reply:
FourierDCT::fftl (then it's listing arguments from file) is not a non-empty list or rectangular array of numeric quantities.
If u haven't tried, u can read this article for help Discrete Fourier Transforms
P.S I've tried running an example and it works perfect 
Hope I've helped you a little bit. And if u solve your problem, I'll be glad to know the answer :)