After playing around with http2 for a couple of servers I am commissioning (not this one yet!) I consistently run into issues with root being unable to log into phpmyadmin.
Logging into mysql manually works perfectly, the password is definitely correct.
If you have the same issue, try this as root:
echo "update user set plugin='' where User='root'; flush privileges;" | mysql --defaults-file=/etc/mysql/debian.cnf mysql
Let me know if it works for you. This post is actually mainly here because I never remember to do that line :P