Warning migrating Matomo WP to On-Premise

Hello,

I am trying to migrate my Matomo WP data to On-Premise according to this instruction: https://plugins.matomo.org/Migration.

However I am getting two warnings:

Warning: Undefined array key “HTTP_HOST” in /httpdocs/…/wp-config.php on line 103
Warning: Undefined array key “HTTP_HOST” in /httpdocs/…/wp-includes/ms-settings.php on line 57

Any idea whats wrong?

Thanks,
Georg