The certificate got lost last time, here it is:
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
Comparing the two certificates above, the /x/crypto/acme: version has an identical extension to my certificate, but the acme.sh is different, as you said.
Both certificates had differing key usage extensions, which I tried adding to my own version, but still same error from Boulder.
Angus