I've found that jenkins-plugin-manager-1.0.1 fails when not used against updates.jenkins.io. For example, the Cloudbees update center fails because that site has no update-center.actual.json file available there. There seems to be is an update-center.json for all of the update centers.

The actual file has pure json content while the non-actual file has a wrapper around the json which is "updateCenter.post( ... json goes here ... );" Someone said that has to do with allowing the file to be more easily compatible with browsers use of javascript.

I don't know if the best thing is to get Cloudbees to add the non-actual file or change the code to remove the wrapper text in in the non-actual file and use it.