Please check as following:
- Has this content source been crawled before? Double check if you are using the correct URL for the issued content source.
- Do crawls for others content sources finish without error? Is there anything special or worth mentioning about this issued one? Check the Event Viewer or ULS logs while crawling for other potential error message on related issue.
- Do an Index Reset via Search Service Application if necessary. After that, run a full crawl again.
See another post with similar issue here: “The SharePoint item being crawled returned an error when requesting data from the web service. ( Error from SharePoint site: Value does not fall within the expected range. )”.
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