I am not sure if the Cloudflare issue is regional or broader. Are you, @dwainzwerg or @simongreenwood willing to test? Set 1.1.1.1 as your DNS, then disconnect / reconnect to make sure applied and see if connectivity check passes?
Note: I think if you report on if you can or can’t ping captiveportal.kuketz.de from any device not just from iodéOS when Cloudflare is the DNS then it would be a good help?