@Ash - Thanks for your response.
Before reindexing I've removed the ISearchable interface on all my controllers and implemented seperate ModuleSearchBase controllers because on my controller I implement an abstract base class so I can't inherit from another abstract class. (no multiple inheritance on classes)
Then I've put the ModuleSearchBase controller class as the businessControllerClass in the dnn file of my module.

I have this in my dnn file:

'<businessControllerClass>DataCore.DAL.SearchControllers.ProjectSearchBaseController</businessControllerClass>'

            '<supportedFeatures>'

              '<supportedFeature type="Searchable" />'

            '</supportedFeatures>'

I've tried to remove a few modules in the result scope for content type list and reindexed the site crawler and now it shows the folowing result in schedule history:
Search: Site Crawler Starting. Content change start time 12/09/2013 15:10
Tabs Indexed: 0
Modules (Metadata) Indexed: 0
Modules (Content) Indexed: 0
Total Items Indexed: 0
Indexing Successful

Normally the site crawler indexes some content but now it states 0. Could this happen because I deleted the ISearchable interface?

@ash - Today I tried it again and miracolously it indexes my modules again. Don't know what happened yesterday. But still no results in my search page. When I look in my log file it states

2013-09-13 07:52:47,938 [Jurgen][Thread:8][ERROR] DotNetNuke.Framework.Reflection - DotNetNuke.Modules.Summary.Components.FeatureController
System.Web.HttpException (0x80004005): Could not load type 'DotNetNuke.Modules.Summary.Components.FeatureController'.
at System.Web.Compilation.BuildManager.GetType(String typeName, Boolean throw , Boolean ignoreCase)
at DotNetNuke.Framework.Reflection.CreateType(String TypeName, String CacheKey, Boolean UseCache, Boolean IgnoreErrors)

I don't use the FeatureController in my modules. Do I have to delete them?

Also the modules where I've put my ModuleSearchBaseController as business controller class doesn't show up in the log file, so I think that's fine then. But why I don't get any resulst when I search?