Remote Plugin Dashboard Fails to Load (502 Bad Gateway) in vCenter 8.0 Enhanced Linked Mode After Upgrade

Products

• VMware vCenter Server

Issue/Introduction

After upgrading or patching a vCenter Server Appliance (VCSA) to 8.0 U3 or later, you may experience a Failed to load Dashboard error or a blank screen when attempting to access a third-party remote plugin (e.g., NetApp SnapCenter for VMware, Dell OMEVV) within the vSphere Client.

This issue specifically occurs in an Enhanced Linked Mode (ELM) environment when you are logged into one vCenter (e.g., Site A) but attempt to access the dashboard of a remote plugin registered to a different vCenter (e.g., Site B).

Additionally, you may observe the following symptoms:

• Browser Developer Tools: Network traces show a 502 Bad Gateway response when the vSphere UI attempts to load the plugin iframe content.
• Envoy Logs: The /var/log/vmware/envoy/envoy.log on the local vCenter (where you are logged in) records SSL handshake failures matching the remote plugin's IP address:

  TLS_error:|268436502:SSL routines:OPENSSL_internal:SSLV3_ALERT_CERTIFICATE_UNKNOWN

Environment

• vCenter Server Appliance (VCSA) 8.0.x
• Enhanced Linked Mode (ELM) topology
• Third-party Remote vSphere Client Plug-ins

Cause

vCenter 8.0 utilizes the Envoy reverse proxy to broker all internal, cross-node connections to remote plugins via strict TLS validation. Following a VCSA upgrade or patch, the Envoy proxy and vSphere UI cache on the node where the plugin is registered may hold onto stale routing data or fail to broadcast updated SSL certificate thumbprints across the ELM mesh.

Consequently, when you attempt to load the remote plugin on a local ELM node, the local Envoy proxy rejects the connection. It does this because the live SSL certificate presented by the remote appliance does not perfectly match the stale thumbprint cached in the proxy's trust tables. Envoy forcefully drops the connection, resulting in a 502 Bad Gateway error in your browser.

Resolution

To resolve this issue, flush the stale proxy cache to force an ELM thumbprint synchronization.

Important: Because of how ELM caching works, you must perform this action on the vCenter node that owns/manages the affected third-party appliance, not necessarily the local vCenter where you are experiencing the error.

1. Identify the specific vCenter Server node where the third-party plugin is registered (e.g., the remote site vCenter).
2. Connect to that remote VCSA via SSH and log in as root.
3. Execute the following command to perform a hard restart of the reverse proxy and vSphere UI services:

   service-control --restart rhttpproxy vsphere-ui

4. Wait 5-10 minutes for the services to fully initialize and for the ELM trust mesh to replicate the updated certificate thumbprints to the other vCenter nodes.
5. Refresh your browser and access the plugin dashboard.

Additional Information

• Simply unregistering and re-registering the plugin from the third-party appliance UI often fails to resolve this issue because it does not force vCenter's Envoy proxy to clear its deeply embedded routing cache.
• If the issue persists after restarting the services on the remote node, you can verify the exact certificate vCenter is receiving by running the following command from the local vCenter SSH session:

  openssl s_client -showcerts -connect <Plugin-Appliance-IP>:<Port> < /dev/null

  Ensure the Subject Alternative Name (SAN) matches the registered FQDN/IP.