How to Fix Error 503 backend Fetch Failed (8 Methods)
How to deal with the "503 Backend fetch failed" Varnish error on Cloudways
Error 503 Backend Fetch Fails Varnish Cache Server - Setup - Magento 2 Beginner Tutorials, Reviews and Discussions
503 Backend Fetch Failed - Error comes up multiple... - Magento Forums
How to gracefully fall back to cache on 5xx responses with Varnish
Dylan Beattie on Twitter: "Visitors to those sites got a cryptic error message: Error 503 Service Unavailable. Guru Mediation: Details: cache-lcy19240-LCY and then a long, meaningless string of numbers. Like this one