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So it shouldn't expire until september.
If I read the recommended posts right this has been an issue since 2016, but for me it only started happen last year.
Do I need to be worried or should I go with what the Proxmox panel tells me?
Read the email very closely and see if something changed about the certificate that you have right now and in the one from the past.
Spoiler
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.