TLDR: I can’t follow my Pixelfed account from a Mastodon user hosted on the same machine.
My YunoHost server
Hardware: VPS bought online
YunoHost version: 11.1.21.4 (stable)
I have access to my server : Through SSH | through the webadmin
Are you in a special context or did you perform some particular tweaking on your YunoHost instance ? : yes
If yes, please explain: Edited Mastodon config to hide subdomain. More details below.
If your request is related to an app, specify its name and version: Pixelfed 0.11.8~ynh5 and Mastodon 4.1.4~ynh1
Description of my issue
Hey, I’m not sure if this is a Pixelfed issue or a Mastodon issue or a Yunohost issue or a me issue. 
I have Mastodon and Pixelfed running on the same server under different subdomains.
Generally, things are fine. If I post a pic on Pixelfed, users from Mastodon can see it and comment and like etc. If I follow a Mastodon user from Pixelfed, I can see pics they post in my feed.
However, I literally can’t see the Pixelfed account from my Mastodon profile on the same server. If I search for the account, I get an error: 422: Mastodon::PrivateNetworkAddressError
If someone on Mastodon boosts a pic I’ve posted on Pixelfed, I can’t see the boost from my Mastodon account at all. It doesn’t appear in their feed from my perspective.
I can search for my Mastodon account from Pixelfed but when I view the profile it only returns the name and bio, no profile pic, no posts, and it lets me follow but doesn’t seem to actually follow (no notification in Mastodon, no posts in the home feed).
I suspect the problem could be a change I made in Mastodon to hide the subdomain. Following the Mastodon documentation I set the local_domain field to the main domain (example.com), and used the web_domain field for the actual domain (toot.example.com), and then added some nginx entries to point webfinger stuff to the Mastodon subdomain.
This is working as expected in Mastodon, other users just see the shorter domain, but I’m wondering if something I’ve done is affecting Pixelfed? Could the nginx entries affect this? If so, can I fix it and still use the shorter domain for Mastodon?