Collabora unauthorized WOPI host

Please, how to diagnose problems according to this?
“likely comes from the loop through public internet. you should check the Nextcloud logs, likely this will be your public IP. this what you need to add as WOPI Allow list… you can later continue with references above to avoid internet loop.”

I went trough all of the Collabora integration guide several times, every suggested check passing ok, but the end result is still the same.

I tested switching off all browser extensions with no effect.
Can’t see anything useful in the logs. I can’t see anything related to your claim “is pretty clear - your CODE server can not access the file stored in Nextcloud… you should track this request in Nextcloud (probably in reverse proxy logs) to understand why it fails…”

I would be grateful for any further advice.

PS: I tried to switch (again) to the integrated server which also doesn’t open any document succesfully (but that never worked for me unlike the docker hosted collabora server).