The request URL – I picked the one from the very first occurrence in logstash – tries to restore the sandbox item to an edit from 2013, the early days of Wikidata. And apparently it succeeded in that, because I can see the invalid title on the item now (permalink); presumably this is causing subsequent errors in links update jobs etc.

I’m guessing either Wikibase or MediaWiki needs to filter out these invalid links when emitting the page links. (I’m not convinced Wikibase should prevent saving them: that wouldn’t solve the problem for existing revisions, and also, the existence of the CommonsLink constraint checker in WikibaseQualityConstraints kinda suggests that it’s intentional that it’s not rejected at a “stricter” level.)

In T405350#11205669, @Lucas_Werkmeister_WMDE wrote:

I’m guessing either Wikibase or MediaWiki needs to filter out these invalid links when emitting the page links.

Probably somewhere in Wikibase’s ImageLinksDataUpdater, I suspect, or perhaps in the core ParserOutput::addImage() that it calls.