A 114 error is normally attributed to a memory error of some kind. If you run the same mfsort statement using the same data files and the same environment variable settings consecutively and it fails one time but not the next then it is most likely something external on your system that is causing the 114. This could be caused by a number of different variables such as available memory, a bad network connection, disk I-O error, etc.
If you run the same mfsort statement using the same data and settings and always get the 114 error then it may be a problem with the data itself or even the command you are using.
After getting the 114 error is it creating an mfdebug.log file in the current directory?
What version of Net Express are you using?