It sounds like one of the run-time files may be missing. Do you have the entire Server for COBOL 5.1 product installed and are your applications linked as dynamic?
Can you run any of the utilities from NX such as rebuild.exe or do you get the same error?
What wrappack level of NX 5.1 are you running? I would recommend that you use the latest wrappack which is 13. Even though NX 5.1 is not certified under Windows 10 the latest wrappack releases shouldn't have a problem, al least in my testing.
Running the procmon tool from SysInternals (now Microsoft) could give you more detail as to what file it cannot find.