Hi @Harley,
This expiration message was for this certificate, issued Apr 3 10:49:00 2017 GMT with serial # 03:c0:45:99:3c:53:7c:c0:7d:69:8b:81:9f:3f:39:0e:93:cd. It only has the domain "www.tail.at" included.
The certificate you've been renewing (Most recent instance here) has both "www.tail.at" and "tail.at" included in it.
Our expiration mail documentation explains why this happens:
We consider a certificate to be renewed if there is a newer certificate with the exact same set of names, regardless of which account created it. If you’ve issued a new certificate that adds or removes a name relative to your old certificate, you will get expiration email about your old certificate. If you check the certificate currently running on your website, and it shows the correct date, no further action is needed.
In this case the certificate being warned about doesn't have the exact same set of names as the newer certificates, so we warn that its close to expiring. In this case since you already have an unexpired certificate that covers that name & one more you can safely ignore the warning.
Hope that helps!