I think the “mail_template” needs to be emptied to use the default one as in the other issue.
api-cli run module/metrics1/configure-module --data '{"grafana_path": "", "mail_from": "user@mail.de", "mail_template": "", "mail_to": ["user@mail.de"], "prometheus_path": ""}'
To apply the config, restart alertmanager:
runagent -m metrics1 systemctl --user restart alertmanager
Now the alert mails shouldn’t be empty anymore.