William,

Thanks for your answer. I checked this, the businessControllerClass definition in the manifest includes the full namespace and there is no typo as well. Is it possible that it does not work because the class derives from ModuleSearchBase?

namespace my.DNN.Modules.xy
{
   public class xyController : ModuleSearchBase, IUpgradeable
   {
      // ... lot of stuff
      public string UpgradeModule(string version)
      {
         // ... see above
      }

      public override IList GetModifiedSearchDocuments(ModuleInfo module, DateTime beginDateUtc)
      {
         // ... see above
      }         
   }
}

The search is working fine, so I did not suspect this constellation.

Happy DNNing!
Michael