On a Proxmox host running:-
apt update
yields:-
Ign:1 https://www.starwindsoftware.com/proxmox master InRelease
Err:5 https://www.starwindsoftware.com/proxmox master Release
404 Not Found [IP: 172.66.42.241 443]
E: The repository 'http://repo.starwind.com/proxmox master Release' no longer has a Release file.
I'm sure it is on the todo list after the website update but just in case, I thought I'd bring it to your attention.
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