List of steps to reproduce (step by step, including full links if applicable):

• https://commons.wikimedia.beta.wmflabs.org/w/api.php?action=query&format=json&export=true&titles=User%3AAJ%2Fcommon.js

What happens?:
Request from (I connect through AMS-IX) via deployment-cache-text06 deployment-cache-text06, Varnish XID 406782011
Error: 503, Backend fetch failed at Mon, 07 Mar 2022 10:59:32 GMT

What should have happened instead?:
200