If your upgrading from an older version of DNN to 5.3.1 (the most stable version of DNN right now) and the the following error when trying to access the Host Settings of your site...

DotNetNuke.Services.Exceptions.ModuleLoadException: ...DesktopModules\Admin\HostSettings\HostSettings.ascx.vb(308): error BC30198: ')' expected.

Then you should...
Check your web.config and make sure the following is in it.  If not, add it.
<compilers>
      <compiler language="vb;vbs;visualbasic;vbscript" type="Microsoft.VisualBasic.VBCodeProvider, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" extension=".vb" warningLevel="4">
        <providerOption name="CompilerVersion" value="v3.5"/>
        <providerOption name="OptionInfer" value="true"/>
        <providerOption name="WarnAsError" value="false"/>
      </compiler>
      <compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CSharp.CSharpCodeProvider,System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" warningLevel="4">
        <providerOption name="CompilerVersion" value="v3.5"/>
        <providerOption name="WarnAsError" value="false"/>
      </compiler>
    </compilers>

That should do it since the erorr is getting thrown on LINQ syntax which is .Net 3.5 specific.  Another fix is to set your site to run under .Net 4.0 from within the IIS Settings to force your site to compile over the newer framework.  But others have reported the following as a solution to this error as well..
<moduleCaching defaultProvider="FileModuleCachingProvider">
       <providers>
         <clear />
         <add name="FileModuleCachingProvider" type="DotNetNuke.Services.ModuleCache.FileProvider, DotNetNuke.Provider.ModuleCaching.FileProvider" providerPath="~\Providers\ModuleCachingProviders\FileModuleCachingProvider\" />
         <add name="MemoryModuleCachingProvider" type="DotNetNuke.Services.ModuleCache.MemoryProvider, DotNetNuke.Provider.ModuleCaching.MemoryProvider" providerPath="~\Providers\ModuleCachingProviders\MemoryModuleCachingProvider\" />
       </providers>
     </moduleCaching>
     <outputCaching defaultProvider="FileModuleCachingProvider">
       <providers>
         <clear />
       </providers>
     </outputCaching>

You will also need to add..
      <section name="moduleCaching" requirePermission="false" type="DotNetNuke.Framework.Providers.ProviderConfigurationHandler, DotNetNuke" />
      <section name="outputCaching" requirePermission="false" type="DotNetNuke.Framework.Providers.ProviderConfigurationHandler, DotNetNuke" />

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Professional DNN Extensions, custom solutions and mobile apps since 2003.
www.OnyakTech.com

Solved by installing a new istance od DNN 5.5.1 Merge my webconfig and the one of the new installation replaced some block of script the main was the follow In My one some line are missing! rmartin

<assemblies>
        <add assembly="Microsoft.VisualBasic, Version=8.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A" />
        <add assembly="System.DirectoryServices, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A" />
        <add assembly="System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A" />
        <add assembly="System.Management, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A" />
        <add assembly="System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089" />
        <add assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
        <add assembly="System.Xml.Linq, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089" />
        <add assembly="System.Data.DataSetExtensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089" />
        <add assembly="System.Data.Linq, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089" />
        <add assembly="System.Web.Abstractions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
      </assemblies>