A few more occurrences today of this error, last one around 13:45 ... just a few minutes short of when we deployed T410610: Extend error reporting context in MediaWiki
Which means with the next error occurrence we at least will know which wiki this is happening on

Ah now we got the wiki - and indeed its broken! https://qichwa.wikibase.cloud/wiki/Qichwabase

So according to https://www.mediawiki.org/wiki/Extension:EmbedVideo
this is one way how you can embed a video on a page: <youtube>https://www.youtube.com/watch?v=eAORm-8b1Eg</youtube>
and I replicated this on the Main Page of one of my wikis on staging: https://deerbase.wikibase.dev/

So I assume this qichwa.wikibase.cloud also embeds a video on their Main Page and this extension is for some reason broken right now in our mediawiki image.

PR: https://github.com/wbstack/mediawiki/pull/510

Merged, deployed and tested on https://qichwa.wikibase.cloud/wiki/Qichwabase