Whenever I have seen this error occur, it was permissions related.  Your hosting provider may have given the correct permissions to the root directory, but those same permissions need to be on every file in the DNN site.

I have reported this error here:

http://support.dotnetnuke.com/issue/ViewIssue.aspx?id=9034&PROJID=2

I also blogged about this here:

http://www.willstrohl.com/Blog/tabid/66/EntryId/278/DNN-Database-Installation-Error-Undefined-Success.aspx

Hopefully, this fixes your problem.

1. please ensure you have ms ajax installed (we recommend installing the 3.5 sp1 version of the .net framework) as it's a requirement for dnn 5.x

2. please check your permissions, this error can occur if the aspnet user does not have the right to read from a file. This can happen if inheritance is not on e.g. you may have set the permissions correctly, then copied across new files and as inheritance is not on, the permissions did not propagate.

3. clear out your local browser cache and cookies - ajax/json errors can occur if script files are corrupted.

4. if you have a proxy cache, please clear it also (same reason as above)

5. ensure static files are allowed to be served in IIS7 (this is the default, but please check)
 

• Go to Control Panel\Programs\Programs and Features and select 'Turn Windows features on or off
• Find and expand Internet Information Services\World Wide Web Services\Common HTTP Features
• ensure the 'Static Content' checkbox is checked

6. please make sure your web.config entries are correct. A bug in dnn 5.0 can cause the web.config to contain blank xmlns i.e.

</system.web>
  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <probing privatePath="bin;bin\HttpModules;bin\Providers;bin\Modules;bin\Support;" />
      <dependentAssembly xmlns="">
        <assemblyIdentity name="System.Web.Extensions" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0" />
      </dependentAssembly>
      <dependentAssembly xmlns="">
        <assemblyIdentity name="System.Web.Extensions.Design" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0" />
      </dependentAssembly>
    </assemblyBinding>
  </runtime>

If you have the xmlns="" in your web.config, add the correct text inside the empty quotes... xmlns="urn:schemas-microsoft-com:asm.v1", which should result in something like this...

</system.web>
  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <probing privatePath="bin;bin\HttpModules;bin\Providers;bin\Modules;bin\Support;" />
      <dependentAssembly xmlns="urn:schemas-microsoft-com:asm.v1">
        <assemblyIdentity name="System.Web.Extensions" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0" />
      </dependentAssembly>
      <dependentAssembly xmlns="urn:schemas-microsoft-com:asm.v1">
        <assemblyIdentity name="System.Web.Extensions.Design" publicKeyToken="31bf3856ad364e35" />
        <bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0" />
      </dependentAssembly>
    </assemblyBinding>
  </runtime>

Cathal