First, sorry that I misread your prior post, the command fails on mysql connection using root user: so the same password should appear in every app installation (that creates a MySQL database).
I was (wrongly) talking about the password that was generated for the MySQL user that is created during every app installation.
So for your problem: it seems your MySQL root password has changed…
- Did you try installing another application?
- Can you connect to MySQL this way?
sudo mysql -uroot -p`sudo cat /etc/yunohost/mysql`