WCF Exception Contract Missmatch , I don't think so - wcf

Question

The server did not provide a meaningful reply this might be caused by
a contract mismath, a premature session shutdown or an internal server
error
Hey, I got an exception after calling a wcf service. I know this exception can get found a lot on stackoverflow, but I have a special behaviour. I also tried nearly all solution which are recommended, without success...
My project does the following: The module is like a normal chat, a wpf and asp usercontrol calls the service every 5 seconds and gets the online users. this normally worked fine on my windows 7 workstation. After I started to develope software on my new workstation with win 8 on it, I got this exception. The IIS Configuration are still the same. After the first user starts to frequently call the service everything works, until a second user calls the same service the exception above shows up.
I set up a testmachine on win 7 and look what happens... Everything works again... I don't have a clue what to do ...
Service web.config:
<?xml version="1.0" encoding="utf-8"?>
<configuration>
<system.diagnostics>
<sources>
<source name="System.ServiceModel" switchName="Information, ActivityTracing" propagateActivity="true">
<listeners>
<add name="xml" />
</listeners>
</source>
<source name="System.ServiceModel.MessageLogging">
<listeners>
<add name="xml" />
</listeners>
</source>
<source name="myUserTraceSource" switchName="Information, ActivityTracing">
<listeners>
<add name="xml" />
</listeners>
</source>
</sources>
<sharedListeners>
<add name="xml" type="System.Diagnostics.XmlWriterTraceListener" initializeData="hi5.PSA.ServiceLog.svclog" />
</sharedListeners>
</system.diagnostics>
<appSettings>
<add key="Log4NetPath" value="config.log4net" />
</appSettings>
<system.web>
<compilation debug="true" targetFramework="4.0">
<assemblies>
<add assembly="System.Data.Entity, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
</assemblies>
</compilation>
<pages controlRenderingCompatibilityVersion="3.5" clientIDMode="AutoID" />
<!-- that many MB should be enough :) -->
<httpRuntime maxRequestLength="2147483647" />
</system.web>
<system.serviceModel>
<bindings>
<basicHttpBinding>
<binding name="BasicHttpBinding_IPmiLookup" />
</basicHttpBinding>
</bindings>
<services>
<service name="SolutionName.ChatService" behaviorConfiguration="chatServiceBehavior">
<endpoint address="" binding="wsDualHttpBinding" contract="SolutionName.contracts.Chat.IChatManager" />
<endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange" />
</service>
</services>
<behaviors>
<serviceBehaviors>
<behavior name="chatServiceBehavior">
<serviceMetadata httpGetEnabled="true" />
<serviceDebug includeExceptionDetailInFaults="true" />
</behavior>
<behavior name="">
<serviceMetadata httpGetEnabled="true" />
<serviceDebug includeExceptionDetailInFaults="true" />
</behavior>
</serviceBehaviors>
</behaviors>
</system.serviceModel>
<system.webServer>
<modules runAllManagedModulesForAllRequests="true"></modules>
</system.webServer>
<connectionStrings>
<add name="MyEntities" connectionString="metadata=res://*/Users.csdl|res://*/Users.ssdl|res://*/Users.msl;provider=System.Data.SqlClient;provider connection string="Data Source=localhost;Initial Catalog=MyDB;Integrated Security=SSPI;MultipleActiveResultSets=True"
" providerName="System.Data.EntityClient" />
</connectionStrings>
</configuration>
Client app.config:
<system.serviceModel>
<bindings>
<wsDualHttpBinding>
<binding name="WSDualHttpBinding_IChatManager" closeTimeout="00:01:00" openTimeout="00:01:00" receiveTimeout="00:10:00" sendTimeout="00:01:00" bypassProxyOnLocal="false" transactionFlow="false" hostNameComparisonMode="StrongWildcard" maxBufferPoolSize="524288" maxReceivedMessageSize="65536" messageEncoding="Text" textEncoding="utf-8" useDefaultWebProxy="true">
<readerQuotas maxDepth="32" maxStringContentLength="8192" maxArrayLength="16384" maxBytesPerRead="4096" maxNameTableCharCount="16384" />
<reliableSession ordered="true" inactivityTimeout="00:10:00" />
<security mode="Message">
<message clientCredentialType="Windows" negotiateServiceCredential="true" algorithmSuite="Default" />
</security>
</binding>
</wsDualHttpBinding>
</bindings>
<client>
<endpoint address="http://localhost/SolutionName/ChatService.svc" binding="wsDualHttpBinding" bindingConfiguration="WSDualHttpBinding_IChatManager" contract="hi5.medview.server.contracts.Chat.IChatManager" name="WSDualHttpBinding_ChatManager">
<identity>
<servicePrincipalName value="host/MyName"/>
</identity>
</endpoint>
</client>
Does anyone has any idea. The thing which makes me crazy is that, the service works fine for the first user who is calling it, but after a second user calls it, the damn thing doesn't work anymore...
Please help...
Thx