Eg when installing Piwik via mydomain:8000, Piwik will probably save trusted_hosts[] = mydomain:8000. As soon as the installation is finished, Piwik will not allow you to log in as the URL does not match. Piwik recommends to set trusted_hosts[] = mydomain in the UI.

I'm not sure what exactly the error is. Either Piwik saves trusted_hosts[] = mydomain:8000 and requires trusted_hosts[] = mydomain or it is the other way around. Or the check whether a host matches doesn't correctly check for the port.