When you’d used scaffolding to create a child theme, I think you probably were able to break it due to something similar to the last time that I had an issue, actually; there was an problem where plugin updates while instant reload was active was causing the site to 504 error, but restarting it would fix it.
https://localwp.com/community/t/504-gateway-timeout/22387/10
Something about the number of files changing rapidly. Respectfully, I think that’s likely a different issue than this one, since we’re not talking here about many files changing at once (and plugin updates work fine for me now; thanks!)