It looks like you have entered some value for forumUrl
which is not a valid relative path. The code works file at my end. The problem comes from the singleThreadPage entered through ForumSettingsControl
singleThreadPage = settings[FORUM_SETTING_SINGLE_THREAD_PAGE];
If the problem persists for you try attaching the VS debugger inside Initialize and GetContentToIndex method inside the provider to see what actually is passed as url.
You could also subscribe to ItemDataBound event of SearchResults control ( the template is located inside Sitefinity/ContorolTemplates/Search folder) and get the DataItem which has a property Url which you can observe.
All the best,
the Telerik team
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