It seems Linode is blocked by Let’s Encrypt itself.
See: Linode policy forbids issuing for name
Unfortunately, @cpu doesn’t mention WHY this policy is in place. Obviously, Let’s Encrypt has all the right to not tell us, but I’m interested why it is blocked. Is it possible for domain name owners to ask for this blockade? Is it something Let’s Encrypt did on its own? There are no CAA records responsible for this blockade…