You notice errors in the DNS accompanied by messages in your logfile like:
and there are requests to an old b.root-server.
Please check if your update is older than 4 years:
grep update /etc/bind/db.root
You might see something like:
last update: February 17, 2016
You can easily fix this bug by doing
wget -O/etc/bind/db.root https://www.internic.net/domain/named.root