@frank I also noticed another issue when setting it up just today on my main site - you need to make sure your author page is not being redirected to your homepage - WP AP needs the profile page to work. Yoast is a culprit for that.
I'm just writing a blog post about the setup, publishing soon hopefully :)
@neil I read your blogpost, great write-up and a clear explanation. Unfortunately I have no success up until now. Next step: Define the correct question I need to ask the webhost (Siteground) to see what is going on. Just asking "Webfinger is not working for my Activitypub plugin to work and be found on the fediverse" might raise some eyebrows...
@frank Great, good to narrow it down - it looked as if it was nginx related?
I noticed an issue for the project where Matthias mentions nginx as a potential configuration problem - https://github.com/pfefferle/wordpress-activitypub/issues/46 - although I'm not sure what the underlying problem is.