Assuming you have a standard deployment with docker compose, go to the directory with the docker-compose.yml file and run this:

 sh

    
docker compose logs lemmy | less

  

You might have to scroll for a bit, you can hit / then type mastodon.social then enter to find occurrences in the logs. Jump to next/previous match by hitting n and N/shift-n respectively.

I was able to fetch the comment without issue (https://feddit.uk/post/42810312/22639417), so it could be something in your setting somewhere. You can try to resolve the comment (go to search in the web UI and past the URL for comment), then after you can run the command above and jump to the end (Alt->) and it should say something about why it failed to resolve.