t can wreak havoc with things like the debian-sys-maint user used to cleanly shutdown mysql.
Although t... change both the root password and the debian-sys-maint password, of course it won't automatically change the expected debian-sys-maint password in /etc/mysql/debian.cnf. In fact, unle... were using from before.
===== Fix the debian sys maint password. =====
So now look up the debian sys m