When accessing config-server dashboard URL, it returned "Whitelabel Error Page" as show in below image.

The config-server service instance is implemented by deploying an application called config-server in org p-spring-cloud-services / space <config-server service instance GUID>. Checking recent logs of config-server app you might see some exception or error. For example,

The error "No such label: master" indicated that there was no "master" branch in configured remote Git repository.
As described in the
document, config-server would try to fetch data from master branch by default if no branch explicitly specified with "label" parameter.
In this case there was only "main" branch in remote Git repository.