That error normally occurs when RORB is unable to or has a problem creating and writing a storm file for the representative hydrograph. The most likely cause of this would be that the storm file name combined with the file path to the storm file is too long as Windows has a limit on the number of characters allowed in a file path.
Another reason could be that you don’t have permissions to write to the folder or something (maybe anti-virus) is stopping it but I think this would be unlikely if RORB is still able to produce the MC output file.
If the length of the file path isn’t the problem would you be able to send me an email with the .catg, .par, any other input files as well as the output files (and possibly storm file) RORB produces when that error occurs? My email is email@example.com.