Unable to add HTTP stores when using Proxy.

We can find "Status code=503" in CDF:
---
AuthManSvr,_#dotNet#_,0,,1,CDF_NET_INFO," CWindowsNetworkServices::TryGetIEProxyConfigForCurrentUser",""
AuthManSvr,_#dotNet#_,0,,1,CDF_NET_INFO," {",""
AuthManSvr,_#dotNet#_,0,,1,CDF_NET_INFO,"     Succeeded",""
AuthManSvr,_#dotNet#_,0,,1,CDF_NET_INFO," }",""
:
AuthManSvr,_#dotNet#_,0,,1,CDF_NET_INFO,"CAuthHttpRequest::GetResponseStatusCode client='AppReceiver WinHttpClient:Receiver.exe:5268:25' object=AuthHttpRequest#9",""
AuthManSvr,_#dotNet#_,0,,1,CDF_NET_INFO,"{",""
AuthManSvr,_#dotNet#_,0,,1,CDF_NET_INFO,"    CAuthHttpRequestImpl::GetResponseStatusCode",""
AuthManSvr,_#dotNet#_,0,,1,CDF_NET_INFO,"    {",""
AuthManSvr,_#dotNet#_,0,,1,CDF_NET_INFO,"        Status code=503",""
AuthManSvr,_#dotNet#_,0,,1,CDF_NET_INFO,"    }",""
AuthManSvr,_#dotNet#_,0,,1,CDF_NET_INFO,"}",""
CitrixReceiver,_#dotNet#_,0,,9,CDF_NET_INFO,"[2017-04-14 13:23:59:185] [1844] [WinHttpClient.cpp:102]     response: status=503
---

503 SERVICE UNAVAILABLE
The server is currently unable to handle the request due to a temporary overload or scheduled maintenance, which will likely be alleviated after some delay.
 

Bypass proxy resolved the issue. Or you can resolve it referring to the following KBs:

• CTX216226 - How to configure Receiver to not use proxy settings set up on IE

• CTX136516 - ​​StoreFront Client Proxy Configuration