It looks like apt (the package manager) is configured to use the backports repository.§ 

When using Yunohost diagnostic tool, you may get the following warning : [WARNING] It looks like apt (the package manager) is configured to use the backports repository. Unless you really know what you are doing, we strongly discourage installing packages from backports, because it's likely to create unstabilities or conflicts on your system.

This may be a bit of a surprise at first, especially when it happens on a fresh install where you haven’t customized APT sources. Fortunately, this forum post is here to explain us the reason for this message, and how to solve the issue. According to Aleks, a Yunohost contributor :

Yeah, some VPS do configure backports. It’s not a huge deal but in the yunohost project we kinda became a bit scared histortically about backports upgrading a shitload of stuff to incompatible versions.

So this is not a big issue, but it could be that using backports lead to some issues with Yunohost. Fortunalely, Aleks tells us how to solve it.

You should run grep -nr backport /etc/apt/sources.list* to identify which line is problematic (in SSH from root or with sudo) Then edit the file with for example nano and comment the line by adding a # in the front

Once done, you can run another diagnostic to ensure everything is fine.