Both the reference URL and the source ID point to a working page instead.
Example: https://www.wikidata.org/wiki/Q1427, date of birth property.
See screenshot below:
Description
Description
Event Timeline
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I suspect that the preview tries to scrape the page, and genealogics.org returns that error page instead.
The StrepHit corpus actually contains http://www.genealogics.org/getperson.php?personID=I00446836&tree=LEO
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There was a problem with URI encoding (&tree=LEO was truncated as it was considered as an extra param by the backend and not as part of the url).
Fixed by this github PR.