I am having the exact same issue, and the same error message (I used dropbox to share the link since google drive and sync.com gave me other errors and copy.com is no longer available. My layout is exactly what you have there. I followed a step by step tutorial to create the Zip file, I uploaded it and then share the link in Technic where it asks but no matter how much I log in or out of Technic, no matter how many times I try to install I cannot. My links are:
Technic Link http://api.technicpack.net/modpack/arcton
Dropbox Link
https://www.dropbox.com/s/6f9sc1f5seog6cz/WitchingHour.zip?dl=0
The exact error I get is:
Error unzipping a file for the following pack: Arcton Attempting to extract file arcton-1.0.zip, but it did not exist. Please consult the modpack author.