I had the same problem upgrading from 5.00.00 to 5.00.01.

The install and update script appeared to run fine, but when you try to access the site on any of the portals, it threw a very non descriptive DNN error on the screen. I found this thread, and checked the web.config, and this is what it looked like....

  <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>

I updated the 2 missing xmlns="urn:schemas-microsoft-com:asm.v1" , saved and did a restart. The first page load threw an error about an object reference, but seemed to go away.          

THANKS!!!!! 

<

<  

<

</

<

<

<

</

</

</