Error extracting modpack .zip

[AlfieSR]

http://api.technicpack.net/modpack/please-fill-in-this-field

I already know that it's not a .rar file, disguised or otherwise

And the zip itself is perfectly readable before upload, on dropbox if downloaded manually, and post-launcher-download, it's only the launcher that's struggling to extract the contents.

It gets as far as extracting Minetweaker (successfully) in the mods folder

While the .zip contains far more files below that mark

The mods folder appears to be the only one missing it's intended contents. The exact mod setup functioned perfectly in testing, with no mods not working or with obvious errors of any kind, so I don't think it's a problem with any of the files contained within the .zip. Additionally, attempting to re-create and re-upload the zip causes the same error, as there is no difference in zip contents between v0.20 and v0.20.1 of the modpack.