[Satellite 6] Capsule installation fails with error: "Proxy capsule.example.com cannot be retrieved: unknown error (response 401)"

Issue

When installing new Capsule server, the installation is failing with the following error:

Proxy capsule.example.com cannot be retrieved: unknown error (response 401)

When checking this deeper in /var/log/foreman-installer/capsule.log, we will find following lines near the end of the file:

[DEBUG 2019-08-30T13:58:05 main] Exit with status code: 6 (signal was 6)
[ERROR 2019-08-30T13:58:05 main] Errors encountered during run:
[ERROR 2019-08-30T13:58:05 main]  /Stage[main]/Foreman_proxy::Register/Foreman_smartproxy[capsule.example.com]: Could not evaluate: Proxy capsule.example.com cannot be retrieved: unknown error (response 401)