Redmine now responds to some successful API requests with 204. See: #32
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https://github.com/redmine/redmine/blob/2a7de5d4ac6b8a7619aef6053ced235e0fdaf799/app/controllers/application_controller.rb#L743-L746
It looks like this changed from Redmine 4.0.9 => 4.1.0:
redmine/redmine@2906328
Tested this for the latest stable version of Redmine at the moment, 4.2.2.