Not sure, as my setup is quite different from yours.
My Nextcloud is installed in a VM and served directly from that VM by Apache (no external reverse proxy), and Colabora is running as a Docker container inside the same VM, also served by the same Apache instance, so basically this setup. I’ve been running it this way for years, and expect for a few minor changes in Apache when they renamed the service and a few environment variables that have been added/changed over the years, I’ve never had any issues.
Did you also read this? Maybe it is of any help: Collabora integration guide
Or maybe it’s some kind name resolving issue (it’s always DNS, or at least related to DNS 
), similiar to the one discussed in this GitHub issue, that came up when I googled the error in your logs: WOPI::CheckFileInfo failed for URI - Broken PIPE · Issue #2906 · nextcloud/richdocuments · GitHub