Bintray is whitelisted in Forefront TMG, but gradle can't access it with a specific dependency

At a place there is a proxy set up to filter out bad content, and it prevents gradle from fetching jar dependencies. I’ve talked with the admin and he whitelisted

Now what’s weird is that from Bintray a lot of dependency gets fetched (I think successfully), but then will not get fetched. Here is the error message:

Could not GET ‘’. Received status code 407 from server: Proxy Authentication Required ( Forefront TMG requires authorization to fulfill the request. Access to the Web Proxy filter is denied. )
Enable Gradle ‘offline mode’ and sync project

Later I talked with the admin and he added again the Bintray url to the whitelist with a final slash, but that doesn’t seemed to help.
What can be the problem? I think it’s not a problem with Forefront, because the rule is set to be top level (as I remember), and if I enable “Enable embedded Maven repository” in Android Studio, it will complain about with the same reason