The new version of the Checkmk update plugin is now online 
This plugin is using the JSON data provided by @baris.leenders under https://download.checkmk.com/stable_downloads.json. It will also be uploaded to the exchange soon.
Before you update from the old version remove it completely (rules and agent_checkmk_download package) please.
@baris.leenders and @martin.hirschvogel THX for your support.