If you are trying to change your domain or remove the :2020 port and the panel URL is showing invalid that means the server cannot access the domain for testing.
This is done using a test at http://your-domain:2020/controller/UrlTest/index, you can check this manually by curling this URL on your server:
curl http://your-domain:2020/controller/UrlTest/index
This should explain the cause of the error. If needed you can also workaround the check if you are certain the domain is configured correctly by editing your /etc/hosts file to point 127.0.0.1 to your new domain.
127.0.0.1 my.domain
