Hey how can we troubleshoot when communities are not being federated?

For example, this community shows that there was an error federating to Lemmy.nz (but not what the error was). Previously it was showing as not allowed.

Here are my settings:

There are other communities listed in this post, though really I'd like the tools to troubleshoot myself: https://lemmy.nz/post/21531352

Any help appreciated 🙂