apex_util.host_url returns http://:

Hi,
as I understand https://docs.oracle.com/en/database/oracle/apex/23.1/aeapi/HOST_URL-Function.html
the sql statement 'select apex_util.host_url from dual;'
should return the URL until the Port, e.g. 'https://testserver.com:443'
In my case it returns just 'http://:'
The statement 'select apex_util.host_url('APEX_PATH') from dual;' returns something like 'http://:https://testserver.com/test/example/'

Something goes wrong there. Can someone help?

We use Apex 23.2 with Oracle 19c, the statements above have been run with SQL*Plus.

Thank You!