iOS: itms-services with proxy authentication

I am using itms-services to install my app over the air. Everything is working fine if I don´t use a proxy. The app installs properly in iOS6 and iOS7.

But if I use a proxy I simply get the message that the app could not be installed. I get no more information. There is also nothing in the device logs. I am sure that the proxy information in the settings are correct.

The proxy uses basic authentication. I used wireshark to have a look at network traffic.

The iPad sends the proxy credentials correctly when requesting the ipa file for installation.

But the proxy still sends an access denied back to the iPad. Once again I am sure that the credentials are correct.

Is it even possible to use itms-services with a proxy that uses proxy authentication? Has someone ever used itms-services with proxy authentication?

I hope someone can help me. Thanks for your help in advance!

It turned out to be a simple problem. Our provider blocked the CA of the server certificate. When the iPad wanted to check if the server certificate is valid it tried to get the root certificate. But the proxy blocked that request. Strange that apple does not give any hint on that in device logs or anywhere else.

So the "proxy authorization required" by the server had nothing to do with the real problem.

The answer to the actual question is: It is possible to use itms-services to install apps OTA with proxy authentication. (We used basic auth in this case?