OK, so different MAME sites provide different zipped packages.
So far, I've encountered 4 different versions :
1 - The one given in the zx-dev forum leads to a "merged" version containing various subfolders with additional files that could belong to some variants, according to the folders names (MegaForce, MegaForce US, etc.). Nexprep absolutely doesn't like them. :p
2 - A "simple" one, with just a 07b.bin file and 4k, 8k and 16 data blocks named 1.xxxx to 18.xxxx, where xxxx are 1 or 2 digits numbers followed by letters ;
3 - The same one, with an additional small html file containing a short message from the guy who hosted it. After removing the html file from the archive, a comparison found it 100% identical to the one just above, so I discarded it. I didn't try to use it with the html file inside.
4 - A very different one, still containing the 07b.bin file, plus 4k, 8k and 16k data blocks named starforc.1 to starforc.18
Since I had an error -1 with the first one, I tried with the second one, and this time, it worked !
I also tried with the 4th one and it worked as well ! A comparison of the resulting starforc.nex files found them 100% identical.
So the 2nd and 4th kinds are the ones to go for. Check the archive contents before trying to build the game. The simpler the better.
HTH. :)