Comment 7 for bug 1922778

Adding a bit more details about this bug on focal-ussuri.

Following log shows that on Bionic the system was able to load modules nf_conntrack_ipv4 and nf_conntrack_ipv6, but on Focal the module name is nf_conntrack. Following logs shows sequence of events before and after series upgrade to Focal.

grep -e "-- Reboot --" -e nf_conntrack -e 'kernel: Linux version' journalctl_--no-pager
Jun 10 12:17:34 ubuntu kernel: Linux version 4.15.0-184-generic (buildd@lcy02-amd64-006) (gcc version 7.5.0 (Ubuntu 7.5.0-3ubuntu1~18.04)) #194-Ubuntu SMP Thu Jun 2 18:54:48 UTC 2022 (Ubuntu 4.15.0-184.194-generic 4.15.18)
Jun 10 12:27:09 testsystem kernel: nf_conntrack version 0.5.0 (65536 buckets, 262144 max)
-- Reboot --
Jun 18 20:04:41 testsystem kernel: Linux version 4.15.0-187-generic (buildd@lcy02-amd64-046) (gcc version 7.5.0 (Ubuntu 7.5.0-3ubuntu1~18.04)) #198-Ubuntu SMP Tue Jun 14 03:23:51 UTC 2022 (Ubuntu 4.15.0-187.198-generic 4.15.18)
Jun 18 20:04:41 testsystem kernel: nf_conntrack version 0.5.0 (65536 buckets, 262144 max)
Jun 18 20:04:41 testsystem systemd-modules-load[1318]: Inserted module 'nf_conntrack_ipv4' <------ successfully loaded on Bionic (kernel 4.15.0-187-generic)
Jun 18 20:04:41 testsystem systemd-modules-load[1318]: Inserted module 'nf_conntrack_ipv6'
-- Reboot --
Jun 18 20:26:47 testsystem kernel: Linux version 5.4.0-120-generic (buildd@lcy02-amd64-006) (gcc version 9.4.0 (Ubuntu 9.4.0-1ubuntu1~20.04.1)) #136-Ubuntu SMP Fri Jun 10 13:40:48 UTC 2022 (Ubuntu 5.4.0-120.136-generic 5.4.189)
Jun 18 20:26:47 testsystem systemd-modules-load[1286]: Failed to find module 'nf_conntrack_ipv4' <-------- failed to load on Focal (kernel 5.4.0-120-generic)
Jun 18 20:26:47 testsystem systemd-modules-load[1286]: Failed to find module 'nf_conntrack_ipv6'
Jun 18 20:26:47 testsystem systemd-sysctl[1299]: Couldn't write '1000000' to 'net/nf_conntrack_max', ignoring: No such file or directory
Jun 18 20:26:47 testsystem systemd-sysctl[1299]: Couldn't write '204800' to 'net/netfilter/nf_conntrack_buckets', ignoring: No such file or directory
Jun 18 20:26:47 testsystem systemd-sysctl[1299]: Couldn't write '1000000' to 'net/netfilter/nf_conntrack_max', ignoring: No such file or directory
Jun 20 11:40:26 testsystem sudo[2800143]: root : TTY=pts/11 ; PWD=/var/log ; USER=root ; COMMAND=/sbin/sysctl net.netfilter.nf_conntrack_max
Jun 20 11:40:26 testsystem sudo[2800145]: root : TTY=pts/11 ; PWD=/var/log ; USER=root ; COMMAND=/sbin/sysctl net.netfilter.nf_conntrack_max

On Focal nf_conntrack module was successfully loaded afterwords (after the systemd-sysctl.service)
grep -Ei '^nf_conntrack ' lsmod
nf_conntrack 139264 10 xt_conntrack,nf_nat,nfnetlink_cttimeout,xt_nat,openvswitch,nf_conntrack_netlink,xt_connmark,xt_CT,nf_conncount,xt_REDIRECT

/etc/modules contains obsloleted module names, hence those failed to load
grep nf_conntrac etc/modules
nf_conntrack_ipv4
nf_conntrack_ipv6

In newer versions nf_conntrack_ipv4 and nf_conntrack_ipv6 was merged into nf_conntrack.

This is related to bug [1]

[1] https://bugs.launchpad.net/charm-neutron-openvswitch/+bug/1851764