Can the host ping our shop, which is used to check validity? ping shop.proxmox.com
As the issue doesn't seem to be that the subscription is void or the like, but rather a technical error in the network, somewhere between your host and our shop.
It would be good to check, if there was a possible change to firewall or another network component at the time this started to fail.
For further help w.r.t. subscriptions please use the Proxmox Customer portal over at https://my.proxmox.com/en - for subscription activation/validity issue you can use this even if your subscription level wouldn't cover enterprise support tickets.
Ok, I found it: it's the change to Proxmox VE itself. I updated from 7 to 8 and with that the (automatic) behavior regarding IPv6 must have changed. A NIC which did not have an IPv6 address before did so now. And as the firewall did not allow such traffic, the verification of the subscription failed with a timeout.
I found it with
$ pvesubscription get
productname: Proxmox VE Community Subscription 1 CPU/year
regdate: 2023-07-30 00:00:00
$ wget https://www.proxmox.com/proxmox-ve/pricing
# resolves to 2a01:7e0:0:424::12 and times out after that...