Error "refresh, failed (no usable master)" when master is present

Description

Slave server doesn't ask for updates after notify is being recived by it.
Error text: error: [example.com.] refresh, failed (no usable master)

The most strange part is that AXFR bootstrapping goes fine.
If zone files are deleted, then slave server is restarted, then files are recreated after notify event (or knotc zone-refresh).

Reproduction

Distro: Gentoo

Master version: 2.7.0
Slave version: 2.7.0

Steps:

1. update SOA record serial on the master
2. perform knotc zone-reload on the master
3. perform knot zone-notify on the master

Full slave log example:

info: [example.com.] notify, incoming, 123.123.123.123@35724: received, serial 2018080900
info: [example.com.] refresh, outgoing, 123.123.123.123@53: remote serial 2018080900, zone is outdated
warning: [example.com.] refresh, remote master not usable
error: [example.com.] refresh, failed (no usable master)

Additional info

Workaround: downgrading slave version to 2.5.7.
Version 2.6.8 is affected too.

More information can be provided on request.