I'm am having a similar problem but i cant download modpacks created by other users. I tried downloading Tyolores RPG but it says error downloading .Zip File (The Cache one). With other packs my computer cant unzip the cache file. i can run the standard packs though. Can someone help with this.
With your problem have you tried deleting the entire file, not just the launcher, and then re downloading it.
To do this on windows go to - %appdata%, Roaming, technic and delete the technic file.
For mac got to - Go, Library, AplicationSupport, technic and delete the technic file.
Hope this helps :)