Cannot add eGift card to Apple Wallet

I'm trying to add an eGift card that I purchased at Cashstar to my iPhone 6S Plus (iOS 9.3.1). It worked the last time I used it (Nov 2015), but won't work now.

If you buy an eGift card from Cashstar (for example, say Panera Bread), you get an email that directs you to a page with the eGift card credentials. It also has the black "Add to Apple Wallet" icon, like on this page: Use Wallet on your iPhone or iPod touch - Apple Support

When I click on the button in Safari, it errors out with a message of "Download Failed; Safari cannot download this file".

Tried contacting Cashstar, who said that they can't help and directed me to Apple.

Anyone else see this error as well, and is this an issue with Cashstar or with my iPhone?