System.DllNotFoundException when calling REST API with Content Encoding gzip

Hi,

calling a REST API with Response Header Content Encoding gzip with RestSharp causes problems. If I exec the code as a simple ConsoleApplication with Visual Studio, there is no problem, so only Unity cannot handle this. Anyone with an idea?

         var client = new RestClient("http://rest.sandbox-immobilienscout24.de");
         var request = new RestRequest("/restapi/api/search/v1.0/search/region", Method.GET);
         client.ClearHandlers();
         request.AddQueryParameter("realestatetype", realestatetype);
         request.AddQueryParameter("geocodes", geocodes);
         request.AddHeader("Accept", "application/json");
         request.AddHeader("Systemauthorization", "blabla");
         IRestResponse response = client.Execute(request);
         Debug.Log("search:" +response.ErrorException);

Response Headers:

 Cache-Control →private
 Content-Encoding →gzip
 Content-Type →application/json;charset=UTF-8
 Date →Sun, 06 Nov 2016 14:21:03 GMT
 Expires →Thu, 01 Jan 1970 01:00:00 CET
 L-IS24-ApiClient →blaKey
 L-IS24-RequestRefnum →06be3d11-6526-4664-acd1-0787dbd7d1f6
 Server →Apache
 Transfer-Encoding →chunked
 Vary →Accept-Encoding

Exception:

 search:System.Net.WebException: MonoPosixHelper ---> System.DllNotFoundException: MonoPosixHelper
   at (wrapper managed-to-native) System.IO.Compression.DeflateStream:CreateZStream (System.IO.Compression.CompressionMode,bool,System.IO.Compression.DeflateStream/UnmanagedReadOrWrite,intptr)
   at System.IO.Compression.DeflateStream..ctor (System.IO.Stream compressedStream, CompressionMode mode, Boolean leaveOpen, Boolean gzip) [0x00000] in <filename unknown>:0 
   at (wrapper remoting-invoke-with-check) System.IO.Compression.DeflateStream:.ctor (System.IO.Stream,System.IO.Compression.CompressionMode,bool,bool)
   at System.IO.Compression.GZipStream..ctor (System.IO.Stream compressedStream, CompressionMode mode, Boolean leaveOpen) [0x00000] in <filename unknown>:0 
   at System.IO.Compression.GZipStream..ctor (System.IO.Stream compressedStream, CompressionMode mode) [0x00000] in <filename unknown>:0 
   at (wrapper remoting-invoke-with-check) System.IO.Compression.GZipStream:.ctor (System.IO.Stream,System.IO.Compression.CompressionMode)
   at System.Net.HttpWebResponse..ctor (System.Uri uri, System.String method, System.Net.WebConnectionData data, System.Net.CookieContainer container) [0x00000] in <filename unknown>:0 
   at (wrapper remoting-invoke-with-check) System.Net.HttpWebResponse:.ctor (System.Uri,string,System.Net.WebConnectionData,System.Net.CookieContainer)
   at System.Net.HttpWebRequest.SetResponseData (System.Net.WebConnectionData data) [0x00000] in <filename unknown>:0 
   --- End of inner exception stack trace ---
   at System.Net.HttpWebRequest.EndGetResponse (IAsyncResult asyncResult) [0x00000] in <filename unknown>:0 
   at System.Net.HttpWebRequest.GetResponse () [0x00000] in <filename unknown>:0 
   at RestSharp.Http.GetRawResponse (System.Net.HttpWebRequest request) [0x00000] in <filename unknown>:0 
 UnityEngine.Debug:Log(Object)
 ApiClient:callSearchApi(String, String) (at Assets/ApiClient.cs:33)
 ApiClient:Start() (at Assets/ApiClient.cs:12)