Can't start BIND9 | Requests repeated too quickly

I'm having a problem when starting BIND9 using sudo systemctl start bind9. I get a random error and when I examine it with sudo systemctl status bind9, I get this:

bind9.service: Failed with result 'exit-code'.
Failed to start BIND Domain Name Server.
bind9.service: Schedule restart job, restart counter is at 20.
Stopped BIND Domain Name Server.
bind9.service: Start request repeated too quickly.

I set DefaultStartLimitIntervalSec and DefaultStartLimitBurst to 40s and 20 respectively on /etc/systemd/system.conf, so I know it's not because the start requests are coming too quickly (I reloaded with sudo systemctl daemon-reload afterwards).

This is what the logs on /var/log/kern.log show but I'm new to Linux so I don't know how to recognize and solve the error based on that:

...apparmor="ALLOWED" operation="open profile="named" name="/proc/cmdline" pid=8226 comm="named" requested_mask="r" denied_mask="r" fsuid=0 ouid=0

...apparmor="ALLOWED" operation="open profile="named" name="/proc/sys/kernel/osrelease" pid=8232 comm="named" requested_mask="r" denied_mask="r" fsuid=0 ouid=0

And this is is my /etc/systemd/system/bind9.service file:

[Unit]
Description=BIND Domain Name Server
Documentation=man:named(8)
After=network.target
Wants=nss-lookup.target
Before=nss-lookup.target

[Service]
Type=forking
EnvironmentFile=-/etc/default/named
ExecStart=/usr/sbin/named $OPTIONS
ExecReload=/usr/sbin/rndc reload
ExecStop=/usr/sbin/rndc stop
Restart=on-failure

[Install]
WantedBy=multi-user.target
Alias=bind9.service

Any help would be very much appreciated!

Thanks!

EDIT: Taking as a guidance the denied_mask=r from the logs, I added to the named profile on apparmor (/etc/apparmor.d/usr.sbin.named), the two path from the errors with the masks where they were denied for (r):

• /proc/cmdline as @{PROC}/cmdline r
• /proc/sys/kernel/osrelease as @{PROC}/sys/kernel/osrelease r

And now the error that I get in the logs is:

...appamor="STATUS" operation="profile_replace" info="same as current profile, skipping" profile="unconfined" name="syslog-ng" pid=9486 common="apparmor_parser"

And stopping or disabling apparmor with sudo systemctl stop|disable apparmor doesn't solve the issue either.

(In case I wasn't supposed to do this modification, please let me know and I'll revert the changes)