Sign in doesn't work on a secondary with relative URL

Ticket https://gitlab.zendesk.com/agent/tickets/163305

The secondary only shows a 502 Error instead of the Sign in page. Git commands are not working. And obviously the secondary node cannot connect to the primary node or viceversa.

Valery's remark: In the logs we see that relative URL part is duplicated in redirect URL

Started GET "/secondary/secondary/oauth/geo/auth?state=4cee769959b3ff17%3AeyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJkYXRhIjp7InJldHVybl90byI6Ii9kdmNzLXNlY29uZGFyeS9kdmNzLXNlY29uZGFyeS9kdmNzLXNlY29...