I've just upgraded to 2.2.0 and noticed that knot failed to start. Upon inspection, I found that:

/usr/lib/systemd/system/knot.service had the following line:

ExecStartPre=/usr/sbin/knotc checkconf

I think this is an error. I had to modify it to:

ExecStartPre=/usr/sbin/knotc conf-check

and then reimport the knot.conf file to get knot to start successfully.