Did you fill all 4 lines ? When you export m2i to m2, you don't have to write in the "output m2i" line.Select, your original m2 (inpout m2), your modified m2i in (inpout m2i), and the path to your new m2 (output m2).
just use lastest M2redux 4.6.1 there nothing confusin about this version
What modifications did you do on your model ? Maybe you broke the structure.I had this problem multiple times for some models, so I had to be tricky and modify them in another way to have what I wanted.