When upgrading NSX Intelligence using NSX Manager Unified Appliance 2.5.x, the upgrade process might sometimes fail at the UI plugin registration step.

Problem

Either a change in the NSX Manager Unified Appliance node's IP address or certificate might have caused the UI plugin registration to fail. After the upgrade failure on the NSX Intelligence CLI session occurs, an upgrade status check reveals the failure occurred, as shown in the following example output from the get upgrade progress-status command. 
nsx-intelligence> get upgrade progress-status
...output snipped...
Upgrade step status:
Name: update_plugin_registration
State: TASK_FAILURE

Errors:
Registration of node failed with status code 400

Status file:
wait_for_intelligence_server: resp_status: 200, body: b'{"results":[{"_links":[],"_revision":0,"_create_user":"admin","_create_time":1581934303820,"_last_modified_user":"admin","_last_modified_time":1581934303820,"_system_owned":false,"id":"default","path":"endpoints/default","end_point_address":"20.20.0.11","protocol":"https","thumbprint":"2d7efed079858e3b9d8b83647f38e48c177d56cbb1ab6669a8d63899be0a5206"}]}'
Node Registration response - 400, b'{"error_code":400,"module_name":"common-service","error_message":"I/O error on GET request for \\\\\\"https://20.20.0.11/api/v1/node/version\\\\\\": No route to host (Host unreachable); nested exception is java.net.NoRouteToHostException: No route to host (Host unreachable)"}'
The following error snippet in the /var/log/resume-upgrade.log file also gives information about the UI plugin registration failure. 
...output snipped...
2020-03-26 15:26:07,291 - Playbook failed at step update_plugin_registration. Run the command 'set debug-mode' followed by 'start upgrade-bundle VMware-NSX-Intelligence-appliance-1.1.0.0.0.15898874 step get_upgrade_task_history' for more info.
{
  "info": "[MUS] UpgradeError: Playbook failed at step update_plugin_registration. Run the command 'set debug-mode' followed by 'start upgrade-bundle VMware-NSX-Intelligence-appliance-1.1.0.0.0.15898874 step get_upgrade_task_history' for more info.",
  "state_text": "CMD_ERROR",
  "state": 2,
  "body": null
}

Cause

This problem can happen if the information about the NSX Manager Unified Appliance node is different from what is saved in the NSX Intelligence appliance. The NSX Manager Unified Appliance node's IP address or certificate might have changed since the last time the NSX Intelligence appliance was installed.

Solution

Update the information about the NSX Manager Unified Appliance node's IP address, certificate, and certificate thumbprint before resuming the NSX Intelligence upgrade process. Use the information provided in the VMware Knowledge Base article https://kb.vmware.com/s/article/78505 to resolve this problem.