Gateway Fails to Start on Tableau Server After Enabling SSL When HTTP Uses Port 443

When enabling SSL on Tableau Server, processes including the Gateway will fail to start if the HTTP port is already configured to use port 443.

One or more services failed to reach their expected state.
node1:
 flowprocessor_0.{build number}: Failed to start. Current status: STATUS_UNAVAILABLE
 gateway_0.{build number}: Failed to start. Current status: DOWN

The following error occurs in the Gateway error.log:

You configured HTTP (80) on the standard HTTPS (443) port.

Cause

The issue occurs from a port conflict during the SSL enablement process.
When enabling SSL on Tableau Server, the Gateway uses port 443 for HTTPS traffic. If the Gateway already uses port 443 for HTTP traffic, Tableau Server attempts to bind both HTTP and HTTPS to the same port. This conflict prevents the Gateway service from starting.