@JuergenAuer I think you’re confusing two different notifications here. There are notifications going out warning users if they are still using the ACMEv1 API for renewals; there are also notifications going out warning users if their recent renewals would have failed the multi-perspective validation process. This is a new process which is separate from the ACMEv1/ACMEv2 issue and is an independent reason that a user’s future renewals could fail.
In this case, @connaxisadmin received the other warning. This could be related to slow DNS updates, or to a firewall that blocks incoming connections from certain IP addresses (like whitelisting an old validation IP address and blocking other port 80 connections, or something).