I know there have been so many threads about this, and still I can’t find one that quite matches my situation and I’ve been pulling my hair out over this for a couple of days now.
I’m running Nextcloud and Collabora using docker-compose on an AWS EC2 instance.
Each has its own subdomain with traffic forwarded from a load balancer (for the convenience of DNS routing, SSL termination and automatic certificate management).
Nextcloud seems to be working just fine.
Collabora admin console is accessible and functional as well.
Configuring the integration through the UI, I get the 
and Collabora Online server is accessible.
However, I cannot open any document.
I get the dreaded screen:
Document loading failed
Failed to load Nextcloud Office - please try again later
Looking at the Collabora logs after attempting to open a document in Nextcloud, I see:
[ docbroker_003 ] WRN
#34 has timed out while requesting
[GET abc.xyz.com/index.php/apps/richdocuments/wopi/files/abc123/contents?access_token=abc123&access_token_ttl=0&permission=edit]
after 30002ms| net/HttpRequest.hpp:1295
…followed by several downstream failures as a result of that timeout.
It seems like Collabora Online simply can’t download the file, but the file is definitely accessible from the Docker container. From within the container I can wget that URL (prepending https:\\) and download the file, no problem.
Any and all suggestions and assistance are greatly appreciated!