Have you tried to recreate the search index? If not, delete the broken one and then try to make a new one identical to the previous. You can also try to go to App_Data->Sitefinity->Search folder and find the folder named as the broken index, then delete its contents. After that go to the backend of the project and re-index.
If the problem persist I would advise to submit a support ticket and send us your project.
the Telerik team
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