When you click "Add" button to add a new credential, it tries to access the following URL:

http://domain/descriptor/com.cloudbees.plugins.credentials.CredentialsSelectHelper/resolver/com.cloudbees.plugins.credentials.CredentialsSelectHelper$SystemContextResolver/provider/com.cloudbees.plugins.credentials.SystemCredentialsProvider$ProviderImpl/context/jenkins/dialog

But it seems to be hardcoding the protocol to HTTP, which causes the request to fail due to mixed http/https content