Apparently this is still an issue. Can't add it from the app or the mobile website (iPhone 13 Pro Max, iOS 15.4).
App behavior is like described in the first post (button does nothing). Website shows up with an error in Safari and Chrome ("The webpage at https://wallet-api.urbanairship.com/v1/pass/adaptive/XXXXXX might be temporarily down or it may have moved permanently to a new web address.").
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