The simplest explanation is that you are now being required to submit credentials in order to add a new node because you have your Cloudera express installation configured to retrieve files from one of Cloudera's repositories.
This past February, Cloudera changed the download location and made CDH, including versions prior to 6.3.3, exclusively available from a private repository. Please see the announcement here: Transition to private repositories for CDH, HDP and HDF. If your organization already has a valid Cloudera subscription, please do read that announcement in full, as there are links in it to documentation on how to upgrade your installation of Cloudera Manager to deal with the repository location change.
If you don't already have a valid Cloudera subscription, you now have two options:
Acquire a valid Cloudera subscription.
Remove your existing installation's dependency on Cloudera's repository for obtaining files when you add a node to the cluster.
You can pursue the first option by contacting the Cloudera Sales Team to initiate the process of obtaining a subscription. The second option involves reconfiguring your installation of Cloudera manager to obtain files from a repository local to your site instead of Cloudera's, as discussed in this thread.
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