This post was originally written by Kiriyashiki, who renamed their GitHub account. I have decided to repost this, since the information in this post is very helpful and I wasn't able to find the new name right away. Credit therefore goes to https://github.com/Kiriyashiki . I (Gab) have also tweaked this post with further information. Also thanks to 2ᵈᵉᵍʳᵉᵉˢGlaceon_ for helping with some edits in this guide!
The 86420 error code is often due to improperly configured networks blocking the P2P connections when going online. Multiple things can cause this, mainly bad NAT or firewalling.
This guide will go through several ways to fix this error. Is it recommended to apply both fixes for minimize the chance of running onto this annoying error. Let's get started. Even if you're not familiar with PCs, this guide will help you fix the error.
Firewalls may block incoming connections to Dolphin. To fix this, we need to allow Dolphin to bypass the firewall.