The sli.do addin for PowerPoint does not seem to like a proxy server with authentication.
13:55:51.775 INF] The addin is connected to Slido API through a proxy. (Proxy server address: "http://xxxxxxx:8012/"). real servername replace by xxxxx
[13:55:52.519 ERR] Unable to create a new app instance on server. Missing proxy authentication (required scheme "BASIC realm="PROXY_INTERNET"").
System.Net.WebException: The remote server returned an error: (407) Proxy Authentication Required.
at System.Net.HttpWebRequest.EndGetRequestStream(IAsyncResult asyncResult, TransportContext& context)
at System.Net.Http.HttpClientHandler.GetRequestStreamCallback(IAsyncResult ar)
So it detects very well the correct proxyserver, but there is no prompt for a login/pwd for the proxyserver, as most programs do in such case.