Still no luck finding an answer that is consistent -- it can be not enough disk space, malformed headers, http2 settings -- I couldn't make it work. My user was using Beaver Builder and she could not edit pages some of the time. The errors were happening randomly, sometimes not.
This error occurred on both of my Webinoly servers at different providers (Google Cloud and Hetzner).
I moved her site to another host and it works perfectly now.
I really want to find out what is causing this, so I can trust my server is functioning properly, so hoping someone has some info on this.