I have no Idea. It worked perfectly for me.
This is what I did:
Copied the zip into my dropbox folder. (The client download thingie. It is equally possible but slightly different with the website)
Waited for it to sync.
Right clicked on the zip and clicked "Share dropbox link"
Copied the URL into the box on the modpack page.
Changed the "www." in the url to "dl."
No problem. There's a gear button on the top right of the launcher screen that opens the main menu, just select 2 or 3 GB in the dropdown. With only 1GB of RAM the worldgen stuff in Hexxit can really lower your frames.
download
I don't really have the dropbox space of the Modpack slots open to host this for you, sorry.
But the important part is that there are no unzipping errors and no block ID conflicts, right?
You're going to have to start a new map. In addition to the prospect of backporting your map to 1.5 which likely wouldn't work, you'd be missing all the ores.
The first thing you should try (if you haven't already) is allocating extra RAM. Both the FTB and Technic launchers default to 1GB, which isn't always enough.