So this looks like issue during sending notification to Nova. You should check in Nova logs now why it returned You 422
The relevant nova-compute log on the node where I try to start the instance looks like this: Nov 27 13:33:24 bc1blade15 nova-compute[4240]: 2020-11-27 13:33:24.870 4240 INFO nova.compute.claims [req-4125d882-0753-4a0a-844c-5a948681ffa1 920e739127a14018a55fb4422b0885e7 0f14905dab5546e0adec2b56c0f6be88 - default default] [instance: 20a23a18-0d13-4aba-b0be-37e243b21336] Claim successful on node bc1blade15.openstack.local Nov 27 13:33:26 bc1blade15 nova-compute[4240]: 2020-11-27 13:33:26.151 4240 INFO nova.virt.libvirt.driver [req-4125d882-0753-4a0a-844c-5a948681ffa1 920e739127a14018a55fb4422b0885e7 0f14905dab5546e0adec2b56c0f6be88 - default default] [instance: 20a23a18-0d13-4aba-b0be-37e243b21336] Creating image Nov 27 13:33:31 bc1blade15 nova-compute[4240]: 2020-11-27 13:33:31.463 4240 INFO os_vif [req-4125d882-0753-4a0a-844c-5a948681ffa1 920e739127a14018a55fb4422b0885e7 0f14905dab5546e0adec2b56c0f6be88 - default default] Successfully plugged vif VIFBridge(active=False,address=fa:16:3e:57:d5:f4,bridge_name='brq497a58ad-e5',has_traffic_filtering=True,id=c2e13a92-86bc-4c8e-ad74-1cad0a6bcffc,network=Network(497a58ad-e57c-4bf1-a1c7-dad58f4795ec),plugin='linux_bridge',port_profile=<?>,preserve_on_delete=False,vif_name='tapc2e13a92-86') Nov 27 13:33:33 bc1blade15 nova-compute[4240]: 2020-11-27 13:33:33.961 4240 INFO nova.compute.manager [req-633ff74b-b16d-4063-9f54-5ee618ea2bb9 - - - - -] [instance: 20a23a18-0d13-4aba-b0be-37e243b21336] VM Started (Lifecycle Event) Nov 27 13:33:34 bc1blade15 nova-compute[4240]: 2020-11-27 13:33:34.104 4240 INFO nova.compute.manager [req-633ff74b-b16d-4063-9f54-5ee618ea2bb9 - - - - -] [instance: 20a23a18-0d13-4aba-b0be-37e243b21336] VM Paused (Lifecycle Event) Nov 27 13:33:34 bc1blade15 nova-compute[4240]: 2020-11-27 13:33:34.388 4240 INFO nova.compute.manager [req-633ff74b-b16d-4063-9f54-5ee618ea2bb9 - - - - -] [instance: 20a23a18-0d13-4aba-b0be-37e243b21336] During sync_power_state the instance has a pending task (spawning). Again, the corresponding neutron-server log: Nov 27 13:39:02 infra1-neutron-server-container-cbf4b105 neutron-server[87]: 2020-11-27 13:39:02.106 87 WARNING neutron.notifiers.nova [-] Nova event: {'server_uuid': '20a23a18-0d13-4aba-b0be-37e243b21336', 'name': 'network-vif-deleted', 'tag': 'c2e13a92-86bc-4c8e-ad74-1cad0a6bcffc', 'status': 'failed', 'code': 422} returned with failed status I also looked in all the logs of the nova related services on the managament node for the instance id but didn't find it. So the nova-compute log seems to indicate that the instance can connect to the bridge successfully?!