I just updated my personal server modpack to 1.6.4, and uploaded it to my web host. The direct link is perfectly valid, I confirmed it many times over from multiple computers. However, every time I try to update my modpack with the new URL, it just gives me the error "You must provide a valid link to your custom zip."
In fact, it gives me the same error when I try to change it back to the same URL that it was before. I also tried creating a new custom modpack and tried the link there, but still no dice.
The pack URL is [removed] but it just shows as 'unavailable' now, since the only way I was able to get the modpack to save was by clearing the URL field entirely.
I'd rather not post the modpack direct link publicly, but I can assure you the link works just fine. I can paste it in my browser address bar and it downloads immediately. No redirection is being used.