The Clavis Historicorum Antiquitatis Posterioris has (apparently without notice) changed the URLs to its database without a redirect from the old URLs.
The IDs apparently are unchanged, so fixing this on #Wikidata was a non-issue (https://www.wikidata.org/wiki/Wikidata:WikiProject_Antiquity/CHAP#Clavis_Historicorum_Antiquitatis_Posterioris_%28CHAP%29)