Indexing Service: Update batch could not be sent to service uri

Hi,

For a website running EPiServer 7.5 Update 35 (7.14.0) we can't get the Indexing service to work.

When we check EPiServer's log files we see the following happen very often:

2016-09-14 04:13:28,012 [494] ERROR SearchSettings: Update batch could not be sent to service uri 'http://www.xxxxxx.com/IndexingService/IndexingService.svc/update/?accesskey=local'. Message: 'The operation has timed out   at System.Net.HttpWebRequest.GetResponse()
   at System.Net.HttpWebRequest.GetResponse()
   at EPiServer.Search.RequestHandler.MakeHttpRequest(String url, NamedIndexingServiceElement namedIndexingServiceElement, String method, Stream postData, Action`1 responseHandler)
   at EPiServer.Search.RequestHandler.SendRequest(SyndicationFeed feed, String namedIndexingService, Collection`1 ids)'
2016-09-14 04:13:28,018 [494] ERROR SearchSettings: Send batch for named index 'serviceName' failed. Items are left in queue.

Currently we have the following settings in the configuration file:

<?xml version="1.0" encoding="utf-8"?><configuration><configSections>
    ...<section name="episerver.search.indexingservice" type="EPiServer.Search.IndexingService.Configuration.IndexingServiceSection, EPiServer.Search.IndexingService" /></configSections>
  ...<runtime><assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1"><probing privatePath="modulesbin" />
	  ...<dependentAssembly><assemblyIdentity name="EPiServer.Search.IndexingService" publicKeyToken="8fe83dea738b45b7" culture="neutral" /><bindingRedirect oldVersion="0.0.0.0-7.5.394.2" newVersion="7.5.394.2" /></dependentAssembly>
	  ...</assemblyBinding></runtime>
  ...<system.serviceModel>
    ...<bindings>
      ...<webHttpBinding><binding name="IndexingServiceCustomBinding" maxBufferPoolSize="1073741824" maxReceivedMessageSize="2147483647" maxBufferSize="2147483647"><readerQuotas maxStringContentLength="10000000" /></binding></webHttpBinding></bindings></system.serviceModel>
  ...<episerver.search active="true"><namedIndexingServices defaultService="serviceName"><services><add name="serviceName" baseUri="http://xxxxxx.com/IndexingService/IndexingService.svc" accessKey="local" /></services></namedIndexingServices><searchResultFilter defaultInclude="false"><providers /></searchResultFilter></episerver.search>
  ..<episerver.search.indexingservice><clients><add name="local" description="local" allowLocal="true" readonly="false" /></clients><namedIndexes defaultIndex="default"><indexes><add name="default" directoryPath="[appDataPath]\Index" readonly="false" /></indexes></namedIndexes></episerver.search.indexingservice><location path="IndexingService/IndexingService.svc"><system.web><httpRuntime maxQueryStringLength="65536" /></system.web><system.webServer><security><requestFiltering><requestLimits maxQueryString="65536" /></requestFiltering></security></system.webServer></location></configuration>

Currently we have the following EPiServer nuget packages installed:

<package id="EPiServer.CMS.Core" version="7.14.0" targetFramework="net452" /><package id="EPiServer.CMS.UI" version="7.14.0" targetFramework="net452" /><package id="EPiServer.CMS.UI.Core" version="7.14.0" targetFramework="net452" /><package id="EPiServer.Framework" version="7.14.0" targetFramework="net452" /><package id="EPiServer.Search" version="7.5.394.2" targetFramework="net452" />

I'm unsure what causes the Indexing service to not working. Does anyone know what could cause this?

Thanks in advance!