As reported by @Dzahn in T114742#1725185:
We still have 3 CRITs in Icinga for "Restbase endpoints health" on aqs and there was a comment next to them linking to this ticket.
https://icinga.wikimedia.org/cgi-bin/icinga/status.cgi?search_string=endpoints
The likely cause is that the service_checker script is set to retrieve the spec from http://localhost:7231/en.wikipedia.org/v1. Alas, AQS exposes only the analytics.wikimedia.org domain.