Hi icing,
I am currrently facing the following issue in testsetup with the the letsencrypt staging env:
When requesting the challenge verification(tls-alpn-01) it can happen that the host is not reachable from the letsencrypt staging
Resulting in an error:
ACME server authz: challenge 'invalid' for ***domain*** at https://acme-staging-v02.api.letsencrypt.org/acme/authz-v3/id . Exact response was: {"identifier":{"type":"dns","value":"***domain***"},"status":"invalid","expires":"2021-10-15T15:48:53Z","challenges":[{"type":"tls-alpn-01","status":"invalid","error":{"type":"urn:ietf:params:acme:error:connection","detail":"Connection refused","status":400},"url":"https://acme-staging-v02.api.letsencrypt.org/acme/chall-v3/id/tCNB2g","token":"HIrAlkWbO-MVwsF-pp92geBF2iXlOt8KE0ZUgtyzG5Ut6w","validationRecord":[{"hostname":"***domain***","port":"443","addressesResolved":["ip"],"addressUsed":"ip"}],"validated":"2021-10-08T15:48:55Z"}]}

Mod md than starts a retry with the same challenge some minutes later ending in the exact same error message again.
Any ideas why? Might it be related to the boulder retry limitation "Boulder does not implement the ability to retry challenges or the Retry-After header"(https://github.com/letsencrypt/boulder/blob/master/docs/acme-divergences.md#section-82)