XZise added a comment.
Okay the first failure with the debug output and it's lists a not existing page. It starts with a lowercase letter which could be also the cause. A second failure could help narrow it down. In both requests to the API the page appears so it doesn't seem to be because of MediaWiki.
Regarding case sensitivity: The siteinfo https://fr.wiktionary.org/w/api.php?action=query&meta=siteinfo reports that the site is case sensitive as well as the namespace https://fr.wiktionary.org/w/api.php?action=query&meta=siteinfo&siprop=namespaces. And I don't know how it can “forget” the case sensitivity. In `Link.parse()` it is checked at all times and if the namespace is not valid it'll crash there.
I might provide a second debug patch which shows the XOR difference (may only of all pages which are in ns 0 and 1). That way we could verify whether the name is capitalized in the other set.
TASK DETAIL https://phabricator.wikimedia.org/T92856
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To: XZise Cc: jayvdb, Aklapper, XZise, pywikipedia-bugs