This can happen when your contact has been removed from their server but did not notice yours.
As a technical solution, we have a script to detect those 404, it's a matter of making it run sometimes. We may not be enough confident in our infrastructure right now to let it fix it automatically.
As a visual workaround, I suppose that if I put a required-name, 404 users would not get shown. I can implement it on every menu items for 16 tokens.
Edit: For a direct solution, I removed https://api.playground.hubl.world/users/davidbolandaae/, https://api.playground.hubl.world/users/alinedetrez/ & https://api.playground.hubl.world/users/francoisearbelot/ from your contact list, as those users does not exists anymore.
This is an issue with the Activity Queue & backlinks. When you remove an user, it sends an activity to any server subscribing to the related object.
For some reason, when this user gets deleted, your server did not properly received this information.
Without a more complex way to sync servers, missing activities could and will ever happen. We have a script to solve 404, but it does not handle contacts, as the urlid by itself is alive.
I can improve the 404 script to handle them for 24 tokens. Then it'll be a matter of running it sometimes.
To solve the visual effect, my proposition above #997 (comment 54128) is still valid.