TRIAGE:

Confirmed rocky-proposed in use.
Confirmed python3-oslo.cache 1.30.1-0ubuntu1.1~cloud0

The bug is in the relation between nova-cloud-controller and memcached. It is using the wrong space IP address.

Steps to recreate:

When running:
openstack availability zone list
^[Unable to establish connection to http://10.244.40.91:8774/v2.1/os-availability-zone/detail: ('Connection aborted.', RemoteDisconnected('Remote end closed connection without response',))


On nova-cloud-conroller/0
root@juju-ab7f6f-18-lxd-5:/var/log/nova# netstat -tn|grep SYN
tcp        0      1 192.168.33.172:39964    192.168.33.163:11211    SYN_SENT

Manual attempt also times out:
root@juju-ab7f6f-18-lxd-5:/var/log/nova# nc -vz 192.168.33.163 11211


ifconfig for nova-cloud-controller/0
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 10.244.41.50  netmask 255.255.248.0  broadcast 10.244.47.255
        inet6 fe80::216:3eff:fe8c:81e2  prefixlen 64  scopeid 0x20<link>
        ether 00:16:3e:8c:81:e2  txqueuelen 1000  (Ethernet)
        RX packets 254198  bytes 295532290 (295.5 MB)
        RX errors 0  dropped 1  overruns 0  frame 0
        TX packets 284144  bytes 109114470 (109.1 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

eth1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 9000
        inet 192.168.33.172  netmask 255.255.255.0  broadcast 192.168.33.255
        inet6 fe80::216:3eff:fe8a:e400  prefixlen 64  scopeid 0x20<link>
        ether 00:16:3e:8a:e4:00  txqueuelen 1000  (Ethernet)
        RX packets 369976  bytes 139660554 (139.6 MB)
        RX errors 0  dropped 1  overruns 0  frame 0
        TX packets 438489  bytes 84210710 (84.2 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 151594  bytes 44616315 (44.6 MB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 151594  bytes 44616315 (44.6 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0


memcached/1 iptables rules
Chain ufw-user-input (1 references)
target     prot opt source               destination         
ACCEPT     tcp  --  10.244.41.55         0.0.0.0/0            tcp dpt:11211
ACCEPT     tcp  --  10.244.41.52         0.0.0.0/0            tcp dpt:11211
ACCEPT     tcp  --  10.244.41.50         0.0.0.0/0            tcp dpt:11211
ACCEPT     tcp  --  192.168.33.180       0.0.0.0/0            tcp dpt:11211
ACCEPT     tcp  --  192.168.33.173       0.0.0.0/0            tcp dpt:11211
ACCEPT     tcp  --  192.168.33.164       0.0.0.0/0            tcp dpt:11211
ACCEPT     tcp  --  192.168.33.167       0.0.0.0/0            tcp dpt:11211
ACCEPT     tcp  --  192.168.33.156       0.0.0.0/0            tcp dpt:11211
ACCEPT     tcp  --  192.168.33.147       0.0.0.0/0            tcp dpt:11211
ACCEPT     tcp  --  192.168.33.151       0.0.0.0/0            tcp dpt:11211
ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:22
DROP       tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:11211

juju run --application nova-cloud-controller -- 'ifconfig |egrep "192.168.33|10.244.41"' 
- Stdout: |2
            inet 10.244.41.52  netmask 255.255.248.0  broadcast 10.244.47.255
            inet 192.168.33.174  netmask 255.255.255.0  broadcast 192.168.33.255
  UnitId: nova-cloud-controller/1
- Stdout: |2
            inet 10.244.41.55  netmask 255.255.248.0  broadcast 10.244.47.255
            inet 192.168.33.177  netmask 255.255.255.0  broadcast 192.168.33.255
  UnitId: nova-cloud-controller/2
- Stdout: |2
            inet 10.244.41.50  netmask 255.255.248.0  broadcast 10.244.47.255
            inet 192.168.33.172  netmask 255.255.255.0  broadcast 192.168.33.255
  UnitId: nova-cloud-controller/0

Has the wrong space entry for all nova-cloud-controllers: i.e. 10.244.41.50 vs 192.168.33.172

Next steps:
check both nova-cloud-controller and memcached charms to make sure they use the correct space for the relation.